diff --git a/README.md b/README.md
index 801ac8a3..0311ffef 100644
--- a/README.md
+++ b/README.md
@@ -54,7 +54,7 @@ the below example for a quick preview of basic library features:
 #include <units/physical/international/velocity.h>
 #include <iostream>
 
-using namespace units;
+using namespace units::physical;
 
 constexpr Velocity auto avg_speed(Length auto d, Time auto t)
 {
@@ -63,7 +63,7 @@ constexpr Velocity auto avg_speed(Length auto d, Time auto t)
 
 int main()
 {
-  using namespace si::literals;
+  using namespace units::physical::si::literals;
   Velocity auto v1 = avg_speed(220q_km, 2q_h);
   Velocity auto v2 = avg_speed(si::length<international::mile>(140), si::time<si::hour>(2));
   Velocity auto v3 = quantity_cast<si::metre_per_second>(v2);
diff --git a/docs/DESIGN.md b/docs/DESIGN.md
index 20816cb1..da77b31f 100644
--- a/docs/DESIGN.md
+++ b/docs/DESIGN.md
@@ -148,7 +148,7 @@ in the design documentation.
 Coming back to units, here are a few examples of unit definitions:
 
 ```cpp
-namespace units::si {
+namespace units::physical::si {
 
 // prefixes
 struct prefix : prefix_family {};
@@ -170,7 +170,7 @@ struct kilometre_per_hour : deduced_unit<kilometre_per_hour, dim_velocity, kilom
 
 }
 
-namespace units::us {
+namespace units::physical::us {
 
 // length
 struct yard : named_scaled_unit<yard, "yd", no_prefix, ratio<9'144, 10'000>, si::metre> {};
@@ -224,7 +224,7 @@ These identifiers provide total ordering of exponents of base dimensions in a de
 The SI physical units system defines 7 base dimensions:
 
 ```cpp
-namespace units::si {
+namespace units::physical::si {
 
 struct dim_length : base_dimension<"L", metre> {};
 struct dim_mass : base_dimension<"M", kilogram> {};
@@ -244,7 +244,7 @@ base unit is needed for the text output of unnamed derived units. Second, there
 one system of physical units. For example CGS definitions look as follows:
 
 ```cpp
-namespace units::cgs {
+namespace units::physical::cgs {
 
 using si::centimetre;
 using si::gram;
@@ -354,7 +354,7 @@ directly define a barye in terms of pascal or vice versa.
 Below are a few examples of derived dimension definitions:
 
 ```cpp
-namespace units::si {
+namespace units::physical::si {
 
 struct dim_velocity : derived_dimension<dim_velocity, metre_per_second,
                                         exp<dim_length, 1>, exp<dim_time, -1>> {};
@@ -448,8 +448,8 @@ Beside adding new elements a few other changes where applied compared to the `st
 
 To simplify writing efficient generic code quantities of each dimension have associated:
 1. Concept (i.e. `units::Length`) that matches a length dimension of any physical systems.
-2. Per-system quantity alias (i.e. `units::si::length<Unit, Rep>` for
-  `units::quantity<units::si::dim_length, Unit, Rep>`).
+2. Per-system quantity alias (i.e. `units::physical::si::length<Unit, Rep>` for
+  `units::quantity<units::physical::si::dim_length, Unit, Rep>`).
 
 Also, to help instantiate quantities with compile-time known values every unit in the library
 has an associated UDL. For example:
@@ -603,9 +603,9 @@ user. If we will get a compilation error message containing `dim_capacitance` in
 the compiler will print the following type instead of the alias:
 
 ```text
-units::detail::derived_dimension_base<units::exp<units::si::dim_electric_current, 2, 1>,
-units::exp<units::si::dim_length, -2, 1>, units::exp<units::si::dim_mass, -1, 1>,
-units::exp<units::si::dim_time, 4, 1> >
+units::detail::derived_dimension_base<units::exp<units::physical::si::dim_electric_current, 2, 1>,
+units::exp<units::physical::si::dim_length, -2, 1>, units::exp<units::physical::si::dim_mass, -1, 1>,
+units::exp<units::physical::si::dim_time, 4, 1> >
 ```
 
 You can notice that even this long syntax was carefully selected to provide quite good user
@@ -760,8 +760,8 @@ struct unknown_dimension : derived_dimension<unknown_dimension<E, ERest...>,
 with this the error log or a debugger breakpoint involving a `temp1` type will include:
 
 ```text
-units::quantity<units::unknown_dimension<units::exp<units::si::dim_length, 2, 1>,
-units::exp<units::si::dim_time, -1, 1> >, units::unknown_coherent_unit, long int>
+units::quantity<units::unknown_dimension<units::exp<units::physical::si::dim_length, 2, 1>,
+units::exp<units::physical::si::dim_time, -1, 1> >, units::unknown_coherent_unit, long int>
 ```
 
 
diff --git a/docs/design.rst b/docs/design.rst
index e13e58e8..d3dc9231 100644
--- a/docs/design.rst
+++ b/docs/design.rst
@@ -6,6 +6,7 @@ Design Deep Dive
     For brevity all the code examples in this chapter assume::
 
         using namespace units;
+        using namespace units::physical;
 
 .. toctree::
     :maxdepth: 2
diff --git a/docs/framework.rst b/docs/framework.rst
index bb3e834a..e33e8d48 100644
--- a/docs/framework.rst
+++ b/docs/framework.rst
@@ -6,6 +6,7 @@ Framework Basics
     For brevity all the code examples in this chapter assume::
 
         using namespace units;
+        using namespace units::physical;
 
 .. toctree::
     :maxdepth: 2
diff --git a/docs/framework/quantities.rst b/docs/framework/quantities.rst
index b83d84d4..00f8c8ea 100644
--- a/docs/framework/quantities.rst
+++ b/docs/framework/quantities.rst
@@ -47,7 +47,7 @@ values the library provides :abbr:`UDL (User Defined Literal)` s for each
 :term:`unit` of every :term:`dimension`. Thanks to them the same code can
 be as simple as::
 
-    using namespace si::literals;
+    using namespace units::physical::si::literals;
     constexpr auto d1 = 123q_km;    // si::length<si::kilometre, std::int64_t>
     constexpr auto d2 = 123.q_km;   // si::length<si::kilometre, long double>
 
@@ -66,7 +66,7 @@ In case the user does not care about the specific unit and representation but
 requires quantity of a concrete dimension than dimension-specific concepts can
 be used::
 
-    using namespace si::literals;
+    using namespace units::physical::si::literals;
     constexpr Length auto d = 123q_km;  // si::length<si::kilometre, std::int64_t>
 
 .. note::
@@ -80,8 +80,8 @@ assume that the user wants to implement an ``avg_speed`` function that will
 be calculating the average speed based on provided distance and duration
 quantities. The usage of such a function can look as follows::
 
-    using namespace si::literals;
-    using namespace international::literals;
+    using namespace units::physical::si::literals;
+    using namespace units::physical::international::literals;
     constexpr Velocity auto v1 = avg_speed(220q_km, 2q_h);
     constexpr Velocity auto v2 = avg_speed(140q_mi, 2q_h);
 
diff --git a/docs/framework/text_output.rst b/docs/framework/text_output.rst
index d6c17f06..2a10e41a 100644
--- a/docs/framework/text_output.rst
+++ b/docs/framework/text_output.rst
@@ -12,8 +12,8 @@ Output Streams
 The easiest way to print a quantity is to provide its object to the output
 stream::
 
-    using namespace si::literals;
-    using namespace international::literals;
+    using namespace units::physical::si::literals;
+    using namespace units::physical::international::literals;
     constexpr Velocity auto v1 = avg_speed(220.q_km, 2q_h);
     constexpr Velocity auto v2 = avg_speed(140.q_mi, 2q_h);
     std::cout << v1 << '\n';  // 110 km/h
diff --git a/docs/framework/units.rst b/docs/framework/units.rst
index 225facd5..47d4a319 100644
--- a/docs/framework/units.rst
+++ b/docs/framework/units.rst
@@ -161,8 +161,8 @@ of ``N/m``):
     :emphasize-lines: 2
 
     struct dim_surface_tension : derived_dimension<dim_surface_tension, newton_per_metre,
-                                                    exp<si::dim_force, 1>,
-                                                    exp<si::dim_length, -1>> {}; // N/m
+                                                   exp<si::dim_force, 1>,
+                                                   exp<si::dim_length, -1>> {}; // N/m
 
 If we defined the above in terms of base units we would end up with
 a ``kg/s²`` derived unit symbol.
@@ -361,7 +361,7 @@ and user should not instantiate it by him/her-self. However the user can sometim
 observe this type in case an unit/dimension conversion expression will end up with an
 unknown/undefined unit type like in the below example::
 
-    using namespace si::literals;
+    using namespace units::physical::si::literals;
 
     Length auto l = 100q_km_per_h * 10q_s;
 
diff --git a/docs/quick_start.rst b/docs/quick_start.rst
index e41e8853..fb59fde2 100644
--- a/docs/quick_start.rst
+++ b/docs/quick_start.rst
@@ -32,7 +32,7 @@ of basic library features::
     #include <units/physical/international/velocity.h>
     #include <iostream>
 
-    using namespace units;
+    using namespace units::physical;
 
     constexpr Velocity auto avg_speed(Length auto d, Time auto t)
     {
@@ -41,7 +41,7 @@ of basic library features::
 
     int main()
     {
-      using namespace si::literals;
+      using namespace units::physical::si::literals;
       Velocity auto v1 = avg_speed(220q_km, 2q_h);
       Velocity auto v2 = avg_speed(si::length<international::mile>(140), si::time<si::hour>(2));
       Velocity auto v3 = quantity_cast<si::metre_per_second>(v2);
diff --git a/docs/reference/systems.rst b/docs/reference/systems.rst
index 77ffb0cc..09e62e0f 100644
--- a/docs/reference/systems.rst
+++ b/docs/reference/systems.rst
@@ -8,7 +8,7 @@ Systems
 SI
 --
 
-.. doxygennamespace:: units::si
+.. doxygennamespace:: units::physical::si
     :members:
     :undoc-members:
     :outline:
diff --git a/docs/use_cases.rst b/docs/use_cases.rst
index 89816b92..b3be890d 100644
--- a/docs/use_cases.rst
+++ b/docs/use_cases.rst
@@ -6,6 +6,7 @@ Use Cases
     For brevity all the code examples in this chapter assume::
 
         using namespace units;
+        using namespace units::physical;
 
 .. toctree::
     :maxdepth: 2
diff --git a/docs/use_cases/legacy_interfaces.rst b/docs/use_cases/legacy_interfaces.rst
index 0edd399c..e689a373 100644
--- a/docs/use_cases/legacy_interfaces.rst
+++ b/docs/use_cases/legacy_interfaces.rst
@@ -21,7 +21,7 @@ pass it to the library's output:
     #include "legacy.h"
     #include <units/physical/si/velocity.h>
 
-    using namespace units;
+    using namespace units::physical;
 
     constexpr Velocity auto avg_speed(Length auto d, Time auto t)
     {
diff --git a/example/alternative_namespaces/acceleration.h b/example/alternative_namespaces/acceleration.h
index 3f104517..404a3187 100644
--- a/example/alternative_namespaces/acceleration.h
+++ b/example/alternative_namespaces/acceleration.h
@@ -9,7 +9,7 @@ namespace experimental{
 namespace acceleration {
 
 template<typename Rep = double>
-using m_per_s2 = units::si::acceleration<units::si::metre_per_second_sq, Rep>;
+using m_per_s2 = units::physical::si::acceleration<units::physical::si::metre_per_second_sq, Rep>;
 
 template<typename Rep = double>
 constexpr m_per_s2<Rep> g{static_cast<Rep>(9.80665)};
diff --git a/example/alternative_namespaces/area.h b/example/alternative_namespaces/area.h
index 610542e7..fb1a9ab9 100644
--- a/example/alternative_namespaces/area.h
+++ b/example/alternative_namespaces/area.h
@@ -6,9 +6,9 @@
 namespace area {
 
 template<typename Rep = double>
-using m2 = units::si::area<units::si::square_metre, Rep>;
+using m2 = units::physical::si::area<units::physical::si::square_metre, Rep>;
 
 template<typename Rep = double>
-using fm2 = units::si::area<units::si::square_femtometre, Rep>;
+using fm2 = units::physical::si::area<units::physical::si::square_femtometre, Rep>;
 
 }  // namespace area
diff --git a/example/alternative_namespaces/box_example.cpp b/example/alternative_namespaces/box_example.cpp
index d18b25bd..d22c9bc6 100644
--- a/example/alternative_namespaces/box_example.cpp
+++ b/example/alternative_namespaces/box_example.cpp
@@ -52,7 +52,7 @@ struct Box {
 
 #include <iostream>
 
-using namespace units::si::literals;
+using namespace units::physical::si::literals;
 
 int main()
 {
diff --git a/example/alternative_namespaces/capacitor_time_curve.cpp b/example/alternative_namespaces/capacitor_time_curve.cpp
index 9577a98f..8ff58195 100644
--- a/example/alternative_namespaces/capacitor_time_curve.cpp
+++ b/example/alternative_namespaces/capacitor_time_curve.cpp
@@ -28,7 +28,7 @@
 #include <iostream>
 
 using namespace units::experimental;
-using namespace units::si::literals;
+using namespace units::physical::si::literals;
 
 int main()
 {
diff --git a/example/alternative_namespaces/clcpp_response.cpp b/example/alternative_namespaces/clcpp_response.cpp
index 567b3100..6d295ed6 100644
--- a/example/alternative_namespaces/clcpp_response.cpp
+++ b/example/alternative_namespaces/clcpp_response.cpp
@@ -24,8 +24,8 @@
 #include "./area.h"
 #include "./units_str.h"
 
-using namespace units::si::literals;
-using namespace units::international;
+using namespace units::physical::si::literals;
+using namespace units::physical::international;
 using namespace units::experimental;
 
 void simple_quantities()
diff --git a/example/alternative_namespaces/conversion_factor.cpp b/example/alternative_namespaces/conversion_factor.cpp
index 900d0552..8e11a66e 100644
--- a/example/alternative_namespaces/conversion_factor.cpp
+++ b/example/alternative_namespaces/conversion_factor.cpp
@@ -40,7 +40,7 @@ inline constexpr std::common_type_t<typename Target::rep, typename Source::rep>
 
 }  // namespace
 
-using namespace units::si::literals;
+using namespace units::physical::si::literals;
 using namespace units::experimental;
 
 int main()
diff --git a/example/alternative_namespaces/density.h b/example/alternative_namespaces/density.h
index 5a9dc60b..870da078 100644
--- a/example/alternative_namespaces/density.h
+++ b/example/alternative_namespaces/density.h
@@ -10,7 +10,7 @@ namespace experimental{
 namespace density {
 
 template<typename Rep = double>
-using kg_per_m3 = units::si::density<units::si::kilogram_per_metre_cub, Rep>;
+using kg_per_m3 = units::physical::si::density<units::physical::si::kilogram_per_metre_cub, Rep>;
 
 }
 
diff --git a/example/alternative_namespaces/force.h b/example/alternative_namespaces/force.h
index a2cefd4d..ad0945e5 100644
--- a/example/alternative_namespaces/force.h
+++ b/example/alternative_namespaces/force.h
@@ -9,7 +9,7 @@ namespace experimental{
 namespace force {
 
 template<typename Rep = double>
-using N = units::si::force<units::si::newton, Rep>;
+using N = units::physical::si::force<units::physical::si::newton, Rep>;
 
 }
 
diff --git a/example/alternative_namespaces/length.h b/example/alternative_namespaces/length.h
index 7410a0be..8d0f94f9 100644
--- a/example/alternative_namespaces/length.h
+++ b/example/alternative_namespaces/length.h
@@ -15,70 +15,70 @@ namespace experimental{
 namespace length {
 
 template<typename Rep = double>
-using m = units::si::length<units::si::metre, Rep>;
+using m = units::physical::si::length<units::physical::si::metre, Rep>;
 
 template<typename Rep = double>
-using mm = units::si::length<units::si::millimetre, Rep>;
+using mm = units::physical::si::length<units::physical::si::millimetre, Rep>;
 
 template<typename Rep = double>
-using fm = units::si::length<units::si::femtometre, Rep>;
+using fm = units::physical::si::length<units::physical::si::femtometre, Rep>;
 
 template<typename Rep = double>
-using km = units::si::length<units::si::kilometre, Rep>;
+using km = units::physical::si::length<units::physical::si::kilometre, Rep>;
 
 template<typename Rep = double>
-using AU = units::si::length<units::si::astronomical_unit, Rep>;
+using AU = units::physical::si::length<units::physical::si::astronomical_unit, Rep>;
 
 template<typename Rep = double>
-using in = units::si::length<units::international::inch, Rep>;
+using in = units::physical::si::length<units::physical::international::inch, Rep>;
 
 template<typename Rep = double>
-using angstrom = units::si::length<units::iau::angstrom, Rep>;
+using angstrom = units::physical::si::length<units::physical::iau::angstrom, Rep>;
 
 template<typename Rep = double>
-using ch = units::si::length<units::imperial::chain, Rep>;
+using ch = units::physical::si::length<units::physical::imperial::chain, Rep>;
 
 template<typename Rep = double>
-using fathom = units::si::length<units::international::fathom, Rep>;
+using fathom = units::physical::si::length<units::physical::international::fathom, Rep>;
 
 template<typename Rep = double>
-using fathom_us = units::si::length<units::us::fathom, Rep>;
+using fathom_us = units::physical::si::length<units::physical::us::fathom, Rep>;
 
 template<typename Rep = double>
-using ft = units::si::length<units::international::foot, Rep>;
+using ft = units::physical::si::length<units::physical::international::foot, Rep>;
 
 template<typename Rep = double>
-using ft_us = units::si::length<units::us::foot, Rep>;
+using ft_us = units::physical::si::length<units::physical::us::foot, Rep>;
 
 template<typename Rep = double>
-using ly = units::si::length<units::iau::light_year, Rep>;
+using ly = units::physical::si::length<units::physical::iau::light_year, Rep>;
 
 template<typename Rep = double>
-using mi = units::si::length<units::international::mile, Rep>;
+using mi = units::physical::si::length<units::physical::international::mile, Rep>;
 
 template<typename Rep = double>
-using mi_naut = units::si::length<units::international::nautical_mile, Rep>;
+using mi_naut = units::physical::si::length<units::physical::international::nautical_mile, Rep>;
 
 template<typename Rep = double>
-using pc = units::si::length<units::iau::parsec, Rep>;
+using pc = units::physical::si::length<units::physical::iau::parsec, Rep>;
 
 template<typename Rep = double>
-using pica_comp = units::si::length<units::typographic::pica_comp, Rep>;
+using pica_comp = units::physical::si::length<units::physical::typographic::pica_comp, Rep>;
 
 template<typename Rep = double>
-using pica_prn = units::si::length<units::typographic::pica_prn, Rep>;
+using pica_prn = units::physical::si::length<units::physical::typographic::pica_prn, Rep>;
 
 template<typename Rep = double>
-using point_comp = units::si::length<units::typographic::point_comp, Rep>;
+using point_comp = units::physical::si::length<units::physical::typographic::point_comp, Rep>;
 
 template<typename Rep = double>
-using point_prn = units::si::length<units::typographic::point_prn, Rep>;
+using point_prn = units::physical::si::length<units::physical::typographic::point_prn, Rep>;
 
 template<typename Rep = double>
-using rd = units::si::length<units::imperial::rod, Rep>;
+using rd = units::physical::si::length<units::physical::imperial::rod, Rep>;
 
 template<typename Rep = double>
-using yd = units::si::length<units::international::yard, Rep>;
+using yd = units::physical::si::length<units::physical::international::yard, Rep>;
 
 
 }  // namespace length
diff --git a/example/alternative_namespaces/mass.h b/example/alternative_namespaces/mass.h
index 39c93c74..57816bc7 100644
--- a/example/alternative_namespaces/mass.h
+++ b/example/alternative_namespaces/mass.h
@@ -10,7 +10,7 @@ namespace experimental{
 namespace mass {
 
 template<typename Rep = double>
-using kg = units::si::mass<units::si::kilogram, Rep>;
+using kg = units::physical::si::mass<units::physical::si::kilogram, Rep>;
 
 }
 
diff --git a/example/alternative_namespaces/time.h b/example/alternative_namespaces/time.h
index a72418aa..f1d6650f 100644
--- a/example/alternative_namespaces/time.h
+++ b/example/alternative_namespaces/time.h
@@ -7,15 +7,15 @@
 namespace q_time {
 
 template<typename Rep = double>
-using s = units::si::time<units::si::second, Rep>;
+using s = units::physical::si::time<units::physical::si::second, Rep>;
 
 template<typename Rep = double>
-using ms = units::si::time<units::si::millisecond, Rep>;
+using ms = units::physical::si::time<units::physical::si::millisecond, Rep>;
 
 template<typename Rep = double>
-using min = units::si::time<units::si::minute, Rep>;
+using min = units::physical::si::time<units::physical::si::minute, Rep>;
 
 template<typename Rep = double>
-using h = units::si::time<units::si::hour, Rep>;
+using h = units::physical::si::time<units::physical::si::hour, Rep>;
 
 }  // namespace time
diff --git a/example/alternative_namespaces/timer.cpp b/example/alternative_namespaces/timer.cpp
index c2a90c1f..3f173f4b 100644
--- a/example/alternative_namespaces/timer.cpp
+++ b/example/alternative_namespaces/timer.cpp
@@ -7,7 +7,7 @@
 */
 
 using namespace units::experimental;
-using namespace units::si::literals;
+using namespace units::physical::si::literals;
 
 int main()
 {
diff --git a/example/alternative_namespaces/voltage.h b/example/alternative_namespaces/voltage.h
index a0807e50..258f597c 100644
--- a/example/alternative_namespaces/voltage.h
+++ b/example/alternative_namespaces/voltage.h
@@ -10,19 +10,19 @@ namespace experimental{
 namespace voltage {
 
 template<typename Rep = double>
-using V = units::si::voltage<units::si::volt, Rep>;
+using V = units::physical::si::voltage<units::physical::si::volt, Rep>;
 
 template<typename Rep = double>
-using mV = units::si::voltage<units::si::millivolt, Rep>;
+using mV = units::physical::si::voltage<units::physical::si::millivolt, Rep>;
 
 template<typename Rep = double>
-using uV = units::si::voltage<units::si::microvolt, Rep>;
+using uV = units::physical::si::voltage<units::physical::si::microvolt, Rep>;
 
 template<typename Rep = double>
-using nV = units::si::voltage<units::si::nanovolt, Rep>;
+using nV = units::physical::si::voltage<units::physical::si::nanovolt, Rep>;
 
 template<typename Rep = double>
-using pV = units::si::voltage<units::si::picovolt, Rep>;
+using pV = units::physical::si::voltage<units::physical::si::picovolt, Rep>;
 
 }  // namespace voltage
 
diff --git a/example/alternative_namespaces/volume.h b/example/alternative_namespaces/volume.h
index a6fa2c39..ff10fcfc 100644
--- a/example/alternative_namespaces/volume.h
+++ b/example/alternative_namespaces/volume.h
@@ -10,7 +10,7 @@ namespace experimental{
 namespace volume {
 
 template<typename Rep = double>
-using m3 = units::si::volume<units::si::cubic_metre, Rep>;
+using m3 = units::physical::si::volume<units::physical::si::cubic_metre, Rep>;
 
 }
 
diff --git a/example/avg_speed.cpp b/example/avg_speed.cpp
index e66c1c5f..7113dfc4 100644
--- a/example/avg_speed.cpp
+++ b/example/avg_speed.cpp
@@ -27,47 +27,47 @@
 
 namespace {
 
-constexpr units::si::velocity<units::si::metre_per_second, int>
-fixed_int_si_avg_speed(units::si::length<units::si::metre, int> d,
-                       units::si::time<units::si::second, int> t)
+using namespace units::physical;
+
+constexpr si::velocity<si::metre_per_second, int>
+fixed_int_si_avg_speed(si::length<si::metre, int> d,
+                       si::time<si::second, int> t)
 {
   return d / t;
 }
 
-constexpr units::si::velocity<units::si::metre_per_second>
-fixed_double_si_avg_speed(units::si::length<units::si::metre> d,
-                          units::si::time<units::si::second> t)
+constexpr si::velocity<si::metre_per_second>
+fixed_double_si_avg_speed(si::length<si::metre> d,
+                          si::time<si::second> t)
 {
   return d / t;
 }
 
 template<typename U1, typename R1, typename U2, typename R2>
-constexpr units::Velocity AUTO si_avg_speed(units::si::length<U1, R1> d,
-                                            units::si::time<U2, R2> t)
+constexpr Velocity AUTO si_avg_speed(si::length<U1, R1> d,
+                                     si::time<U2, R2> t)
 {
   return d / t;
 }
 
-constexpr units::Velocity AUTO avg_speed(units::Length AUTO d, units::Time AUTO t)
+constexpr Velocity AUTO avg_speed(Length AUTO d, Time AUTO t)
 {
   return d / t;
 }
 
-template<units::Length D, units::Time T, units::Velocity V>
+template<Length D, Time T, Velocity V>
 void print_result(D distance, T duration, V velocity)
 {
-  const auto result_in_kmph = units::quantity_cast<units::si::velocity<units::si::kilometre_per_hour>>(velocity);
+  const auto result_in_kmph = units::quantity_cast<si::velocity<si::kilometre_per_hour>>(velocity);
   std::cout << "Average speed of a car that makes " << distance << " in "
             << duration << " is " << result_in_kmph << ".\n";
 }
 
 void example()
 {
-  using namespace units;
-
   // SI (int)
   {
-    using namespace units::si::literals;
+    using namespace units::physical::si::literals;
     constexpr Length AUTO distance = 220q_km;      // constructed from a UDL
     constexpr si::time<si::hour, int> duration(2); // constructed from a value
 
@@ -81,7 +81,7 @@ void example()
 
   // SI (double)
   {
-    using namespace units::si::literals;
+    using namespace units::physical::si::literals;
     constexpr Length AUTO distance = 220.q_km;  // constructed from a UDL
     constexpr si::time<si::hour> duration(2);   // constructed from a value
 
@@ -97,7 +97,7 @@ void example()
 
   // Customary Units (int)
   {
-    using namespace units::international::literals;
+    using namespace units::physical::international::literals;
     constexpr Length AUTO distance = 140q_mi;      // constructed from a UDL
     constexpr si::time<si::hour, int> duration(2); // constructed from a value
 
@@ -113,7 +113,7 @@ void example()
 
   // Customary Units (double)
   {
-    using namespace units::international::literals;
+    using namespace units::physical::international::literals;
     constexpr Length AUTO distance = 140q_mi; // constructed from a UDL
     constexpr si::time<si::hour> duration(2); // constructed from a value
 
@@ -131,7 +131,7 @@ void example()
 
   // CGS (int)
   {
-    using namespace units::cgs::literals;
+    using namespace units::physical::cgs::literals;
     constexpr Length AUTO distance = 22'000'000q_cm;  // constructed from a UDL
     constexpr cgs::time<si::hour, int> duration(2);   // constructed from a value
 
@@ -150,7 +150,7 @@ void example()
 
   // CGS (double)
   {
-    using namespace units::cgs::literals;
+    using namespace units::physical::cgs::literals;
     constexpr Length AUTO distance = 22'000'000q_cm; // constructed from a UDL
     constexpr cgs::time<si::hour> duration(2);       // constructed from a value
 
diff --git a/example/box_example.cpp b/example/box_example.cpp
index 1bc7a5ba..2329a287 100644
--- a/example/box_example.cpp
+++ b/example/box_example.cpp
@@ -11,8 +11,8 @@
 
 namespace {
 
-using namespace units;
-using namespace units::si::literals;
+using namespace units::physical;
+using namespace units::physical::si::literals;
 
 using m = si::metre;
 using kg = si::kilogram;
diff --git a/example/capacitor_time_curve.cpp b/example/capacitor_time_curve.cpp
index 4281ff0f..04186e78 100644
--- a/example/capacitor_time_curve.cpp
+++ b/example/capacitor_time_curve.cpp
@@ -29,8 +29,8 @@
 
 int main()
 {
-  using namespace units;
-  using namespace units::si;
+  using namespace units::physical;
+  using namespace units::physical::si;
 
   std::cout << "mp-units capacitor time curve example...\n";
   std::cout.setf(std::ios_base::fixed, std::ios_base::floatfield);
diff --git a/example/clcpp_response.cpp b/example/clcpp_response.cpp
index d5951497..c117ed25 100644
--- a/example/clcpp_response.cpp
+++ b/example/clcpp_response.cpp
@@ -33,11 +33,11 @@ using namespace units;
 
 void simple_quantities()
 {
-  using namespace units::si;
-  using namespace units::international;
+  using namespace units::physical::si;
+  using namespace units::physical::international;
 
   using distance = length<metre>;
-  using duration = si::time<second>;
+  using duration = physical::si::time<second>;
 
   constexpr distance km = 1.0q_km;
   constexpr distance miles = 1.0q_mi;
@@ -57,8 +57,9 @@ void simple_quantities()
 
 void quantities_with_typed_units()
 {
-  using namespace units::si;
-  using namespace units::international;
+  using namespace units::physical;
+  using namespace units::physical::si;
+  using namespace units::physical::international;
 
   constexpr length<kilometre> km = 1.0q_km;
   constexpr length<mile> miles = 1.0q_mi;
@@ -104,7 +105,7 @@ void quantities_with_typed_units()
 
 void calcs_comparison()
 {
-  using namespace units::si;
+  using namespace units::physical::si;
 
   std::cout << "\nA distinct unit for each type is efficient and accurate\n"
                "when adding two values of the same very big\n"
diff --git a/example/conversion_factor.cpp b/example/conversion_factor.cpp
index 82e71e30..3f9ed02d 100644
--- a/example/conversion_factor.cpp
+++ b/example/conversion_factor.cpp
@@ -41,7 +41,7 @@ inline constexpr std::common_type_t<typename Target::rep, typename Source::rep>
 
 int main()
 {
-  using namespace units::si;
+  using namespace units::physical::si;
 
   std::cout << "conversion factor in mp-units...\n\n";
 
diff --git a/example/hello_units.cpp b/example/hello_units.cpp
index dc21299a..51ba2ff3 100644
--- a/example/hello_units.cpp
+++ b/example/hello_units.cpp
@@ -25,7 +25,7 @@
 #include <units/format.h>
 #include <iostream>
 
-using namespace units;
+using namespace units::physical;
 
 constexpr Velocity AUTO avg_speed(Length AUTO d, Time AUTO t)
 {
@@ -34,7 +34,7 @@ constexpr Velocity AUTO avg_speed(Length AUTO d, Time AUTO t)
 
 int main()
 {
-  using namespace si::literals;
+  using namespace units::physical::si::literals;
   Velocity AUTO v1 = avg_speed(220q_km, 2q_h);
   Velocity AUTO v2 = avg_speed(si::length<international::mile>(140), si::time<si::hour>(2));
   Velocity AUTO v3 = quantity_cast<si::metre_per_second>(v2);
diff --git a/example/kalman_filter-alpha_beta_filter_example2.cpp b/example/kalman_filter-alpha_beta_filter_example2.cpp
index e6b777ab..f2c6b893 100644
--- a/example/kalman_filter-alpha_beta_filter_example2.cpp
+++ b/example/kalman_filter-alpha_beta_filter_example2.cpp
@@ -20,7 +20,7 @@ struct state_variable {
   Q predicted_next_state;
 };
 
-using namespace units::si;
+using namespace units::physical::si;
 
 constexpr auto radar_transmit_interval = 5.0q_s;
 constexpr double kalman_range_gain = 0.2;
diff --git a/example/linear_algebra.cpp b/example/linear_algebra.cpp
index 76fdf2c8..079f6e7c 100644
--- a/example/linear_algebra.cpp
+++ b/example/linear_algebra.cpp
@@ -55,8 +55,8 @@ std::ostream& operator<<(std::ostream& os, const std::experimental::math::matrix
 
 
 using namespace std::experimental::math;
-using namespace units;
-using namespace si::literals;
+using namespace units::physical;
+using namespace units::physical::si::literals;
 
 template<typename Rep = double>
 using vector = fs_vector<Rep, 3>;
@@ -201,10 +201,10 @@ void matrix_of_quantity_tests()
   matrix_of_quantity_divide_by_scalar();
 }
 
-template<Unit U = si::metre, Scalar Rep = double>
+template<units::Unit U = si::metre, units::Scalar Rep = double>
 using length_v = si::length<U, vector<Rep>>;
 
-template<Unit U = si::newton, Scalar Rep = double>
+template<units::Unit U = si::newton, units::Scalar Rep = double>
 using force_v = si::force<U, vector<Rep>>;
 
 void quantity_of_vector_add()
diff --git a/example/measurement.cpp b/example/measurement.cpp
index 8ca3d03f..07db5f86 100644
--- a/example/measurement.cpp
+++ b/example/measurement.cpp
@@ -157,7 +157,7 @@ namespace {
 
 void example()
 {
-  using namespace units;
+  using namespace units::physical;
 
   const auto a = si::acceleration<si::metre_per_second_sq, measurement<double>>(measurement(9.8, 0.1));
   const auto t = si::time<si::second, measurement<double>>(measurement(1.2, 0.1));
diff --git a/example/total_energy.cpp b/example/total_energy.cpp
index 79d59e52..eab1dd0b 100644
--- a/example/total_energy.cpp
+++ b/example/total_energy.cpp
@@ -30,7 +30,7 @@
 
 namespace {
 
-using namespace units;
+using namespace units::physical;
 
 Energy AUTO total_energy(Momentum AUTO p, Mass AUTO m, Velocity AUTO c)
 {
@@ -39,7 +39,7 @@ Energy AUTO total_energy(Momentum AUTO p, Mass AUTO m, Velocity AUTO c)
 
 void si_example()
 {
-  using namespace si;
+  using namespace units::physical::si;
   using GeV = gigaelectronvolt;
 
   constexpr Velocity AUTO c = si2019::speed_of_light<>;
@@ -70,7 +70,7 @@ void si_example()
 
 void natural_example()
 {
-  using namespace natural;
+  using namespace units::physical::natural;
   using GeV = gigaelectronvolt;
 
   constexpr Velocity AUTO c = speed_of_light<>;
diff --git a/example/unknown_dimension.cpp b/example/unknown_dimension.cpp
index 3af27ac9..6b213871 100644
--- a/example/unknown_dimension.cpp
+++ b/example/unknown_dimension.cpp
@@ -25,23 +25,24 @@
 
 namespace {
 
-template<units::Length D, units::Time T>
-constexpr units::Velocity AUTO avg_speed(D d, T t)
+template<units::physical::Length D, units::physical::Time T>
+constexpr units::physical::Velocity AUTO avg_speed(D d, T t)
 {
   return d / t;
 }
 
 void example()
 {
-  using namespace units::si::literals;
+  using namespace units::physical;
+  using namespace units::physical::si::literals;
 
-  units::Length AUTO d1 = 123q_m;
-  units::Time AUTO t1 = 10q_s;
-  units::Velocity AUTO v1 = avg_speed(d1, t1);
+  Length AUTO d1 = 123q_m;
+  Time AUTO t1 = 10q_s;
+  Velocity AUTO v1 = avg_speed(d1, t1);
 
   auto temp1 = v1 * 50q_m;  // produces intermediate unknown dimension with 'unknown_coherent_unit' as its 'coherent_unit'
-  units::Velocity AUTO v2 = temp1 / 100q_m; // back to known dimensions again
-  units::Length AUTO d2 = v2 * 60q_s;
+  Velocity AUTO v2 = temp1 / 100q_m; // back to known dimensions again
+  Length AUTO d2 = v2 * 60q_s;
 
   std::cout << "d1 = " << d1 << '\n';
   std::cout << "t1 = " << t1 << '\n';
diff --git a/src/include/units/data/bitrate.h b/src/include/units/data/bitrate.h
index e894c59d..fa2068af 100644
--- a/src/include/units/data/bitrate.h
+++ b/src/include/units/data/bitrate.h
@@ -30,13 +30,13 @@
 namespace units::data {
 
 struct bit_per_second : unit<bit_per_second> {};
-struct dim_bitrate : derived_dimension<dim_bitrate, bit_per_second, exp<dim_information, 1>, exp<si::dim_time, -1>> {};
+struct dim_bitrate : derived_dimension<dim_bitrate, bit_per_second, exp<dim_information, 1>, exp<physical::si::dim_time, -1>> {};
 
-struct kibibit_per_second : deduced_unit<kibibit_per_second, dim_bitrate, kibibit, si::second> {};
-struct mebibit_per_second : deduced_unit<mebibit_per_second, dim_bitrate, mebibit, si::second> {};
-struct gibibit_per_second : deduced_unit<gibibit_per_second, dim_bitrate, gibibit, si::second> {};
-struct tebibit_per_second : deduced_unit<tebibit_per_second, dim_bitrate, tebibit, si::second> {};
-struct pebibit_per_second : deduced_unit<pebibit_per_second, dim_bitrate, pebibit, si::second> {};
+struct kibibit_per_second : deduced_unit<kibibit_per_second, dim_bitrate, kibibit, physical::si::second> {};
+struct mebibit_per_second : deduced_unit<mebibit_per_second, dim_bitrate, mebibit, physical::si::second> {};
+struct gibibit_per_second : deduced_unit<gibibit_per_second, dim_bitrate, gibibit, physical::si::second> {};
+struct tebibit_per_second : deduced_unit<tebibit_per_second, dim_bitrate, tebibit, physical::si::second> {};
+struct pebibit_per_second : deduced_unit<pebibit_per_second, dim_bitrate, pebibit, physical::si::second> {};
 
 template<typename T>
 concept Bitrate = QuantityOf<T, dim_bitrate>;
diff --git a/src/include/units/data/prefixes.h b/src/include/units/data/prefixes.h
index 4e6e683a..241534bc 100644
--- a/src/include/units/data/prefixes.h
+++ b/src/include/units/data/prefixes.h
@@ -35,4 +35,4 @@ struct tebi : units::prefix<tebi, prefix, "Ti", ratio<        1'099'511'627'776>
 struct pebi : units::prefix<pebi, prefix, "Pi", ratio<    1'125'899'906'842'624>> {};
 struct exbi : units::prefix<exbi, prefix, "Ei", ratio<1'152'921'504'606'846'976>> {};
 
-}  // namespace units::si
+}  // namespace units::data
diff --git a/src/include/units/physical/cgs/acceleration.h b/src/include/units/physical/cgs/acceleration.h
index 38375002..232310c1 100644
--- a/src/include/units/physical/cgs/acceleration.h
+++ b/src/include/units/physical/cgs/acceleration.h
@@ -26,7 +26,7 @@
 #include <units/physical/cgs/velocity.h>
 #include <units/quantity.h>
 
-namespace units::cgs {
+namespace units::physical::cgs {
 
 struct gal : named_unit<gal, "Gal", si::prefix> {};
 struct dim_acceleration : physical::dim_acceleration<dim_acceleration, gal, dim_length, dim_time> {};
@@ -42,4 +42,4 @@ constexpr auto operator"" q_Gal(long double l) { return acceleration<gal, long d
 
 }  // namespace literals
 
-}  // namespace units::cgs
+}  // namespace units::physical::cgs
diff --git a/src/include/units/physical/cgs/area.h b/src/include/units/physical/cgs/area.h
index e9e0f74d..1f52edab 100644
--- a/src/include/units/physical/cgs/area.h
+++ b/src/include/units/physical/cgs/area.h
@@ -27,7 +27,7 @@
 #include <units/physical/si/area.h>
 #include <units/quantity.h>
 
-namespace units::cgs {
+namespace units::physical::cgs {
 
 using si::square_centimetre;
 
@@ -44,4 +44,4 @@ constexpr auto operator"" q_cm2(long double l) { return area<square_centimetre,
 
 }
 
-}  // namespace units::cgs
+}  // namespace units::physical::cgs
diff --git a/src/include/units/physical/cgs/energy.h b/src/include/units/physical/cgs/energy.h
index 61dd495e..9c290898 100644
--- a/src/include/units/physical/cgs/energy.h
+++ b/src/include/units/physical/cgs/energy.h
@@ -27,7 +27,7 @@
 #include <units/physical/si/prefixes.h>
 #include <units/quantity.h>
 
-namespace units::cgs {
+namespace units::physical::cgs {
 
 struct erg : named_unit<erg, "erg", si::prefix> {};
 
@@ -44,4 +44,4 @@ constexpr auto operator"" q_erg(long double l) { return energy<erg, long double>
 
 }  // namespace literals
 
-}  // namespace units::cgs
+}  // namespace units::physical::cgs
diff --git a/src/include/units/physical/cgs/force.h b/src/include/units/physical/cgs/force.h
index 19644b16..ec3e2d7e 100644
--- a/src/include/units/physical/cgs/force.h
+++ b/src/include/units/physical/cgs/force.h
@@ -28,7 +28,7 @@
 #include <units/physical/si/prefixes.h>
 #include <units/quantity.h>
 
-namespace units::cgs {
+namespace units::physical::cgs {
 
 struct dyne : named_unit<dyne, "dyn", si::prefix> {};
 
@@ -45,4 +45,4 @@ constexpr auto operator"" q_dyn(long double l) { return force<dyne, long double>
 
 }  // namespace literals
 
-}  // namespace units::cgs
+}  // namespace units::physical::cgs
diff --git a/src/include/units/physical/cgs/length.h b/src/include/units/physical/cgs/length.h
index 2584cfe4..c5d82468 100644
--- a/src/include/units/physical/cgs/length.h
+++ b/src/include/units/physical/cgs/length.h
@@ -26,7 +26,7 @@
 #include <units/physical/si/length.h>
 #include <units/quantity.h>
 
-namespace units::cgs {
+namespace units::physical::cgs {
 
 using si::centimetre;
 
@@ -43,4 +43,4 @@ constexpr auto operator"" q_cm(long double l) { return length<centimetre, long d
 
 }
 
-}  // namespace units::cgs
+}  // namespace units::physical::cgs
diff --git a/src/include/units/physical/cgs/mass.h b/src/include/units/physical/cgs/mass.h
index e372e198..1ad0456b 100644
--- a/src/include/units/physical/cgs/mass.h
+++ b/src/include/units/physical/cgs/mass.h
@@ -26,7 +26,7 @@
 #include <units/physical/si/mass.h>
 #include <units/quantity.h>
 
-namespace units::cgs {
+namespace units::physical::cgs {
 
 using si::gram;
 
@@ -43,4 +43,4 @@ constexpr auto operator"" q_g(long double l) { return mass<gram, long double>(l)
 
 }
 
-}  // namespace units::cgs
+}  // namespace units::physical::cgs
diff --git a/src/include/units/physical/cgs/power.h b/src/include/units/physical/cgs/power.h
index 06981b01..6a1c9c2e 100644
--- a/src/include/units/physical/cgs/power.h
+++ b/src/include/units/physical/cgs/power.h
@@ -27,7 +27,7 @@
 #include <units/physical/si/prefixes.h>
 #include <units/quantity.h>
 
-namespace units::cgs {
+namespace units::physical::cgs {
 
 struct erg_per_second : unit<erg_per_second> {};
 
@@ -44,4 +44,4 @@ constexpr auto operator"" q_erg_per_s(long double l) { return power<erg_per_seco
 
 }  // namespace literals
 
-}  // namespace units::cgs
+}  // namespace units::physical::cgs
diff --git a/src/include/units/physical/cgs/pressure.h b/src/include/units/physical/cgs/pressure.h
index 33a0a2bd..0c07b3dc 100644
--- a/src/include/units/physical/cgs/pressure.h
+++ b/src/include/units/physical/cgs/pressure.h
@@ -28,7 +28,7 @@
 #include <units/physical/si/prefixes.h>
 #include <units/quantity.h>
 
-namespace units::cgs {
+namespace units::physical::cgs {
 
 struct barye : named_unit<barye, "Ba", si::prefix> {};
 
@@ -45,4 +45,4 @@ constexpr auto operator"" q_Ba(long double l) { return pressure<barye, long doub
 
 }  // namespace literals
 
-}  // namespace units::cgs
+}  // namespace units::physical::cgs
diff --git a/src/include/units/physical/cgs/time.h b/src/include/units/physical/cgs/time.h
index c3ed5f77..1b5df90a 100644
--- a/src/include/units/physical/cgs/time.h
+++ b/src/include/units/physical/cgs/time.h
@@ -26,7 +26,7 @@
 #include <units/physical/si/time.h>
 #include <units/quantity.h>
 
-namespace units::cgs {
+namespace units::physical::cgs {
 
 using si::second;
 
@@ -39,4 +39,4 @@ using si::literals::operator"" q_s;
 
 }
 
-}  // namespace units::cgs
+}  // namespace units::physical::cgs
diff --git a/src/include/units/physical/cgs/velocity.h b/src/include/units/physical/cgs/velocity.h
index 8a96f070..33387715 100644
--- a/src/include/units/physical/cgs/velocity.h
+++ b/src/include/units/physical/cgs/velocity.h
@@ -27,7 +27,7 @@
 #include <units/physical/cgs/time.h>
 #include <units/quantity.h>
 
-namespace units::cgs {
+namespace units::physical::cgs {
 
 struct centimetre_per_second : unit<centimetre_per_second> {};
 struct dim_velocity : physical::dim_velocity<dim_velocity, centimetre_per_second, dim_length, dim_time> {};
@@ -43,4 +43,4 @@ constexpr auto operator"" q_cm_per_s(long double l) { return velocity<centimetre
 
 }  // namespace literals
 
-}  // namespace units::cgs
+}  // namespace units::physical::cgs
diff --git a/src/include/units/physical/dimensions.h b/src/include/units/physical/dimensions.h
index 25253822..e3ff96c9 100644
--- a/src/include/units/physical/dimensions.h
+++ b/src/include/units/physical/dimensions.h
@@ -27,9 +27,7 @@
 #include <units/quantity.h>
 #include <units/unit.h>
 
-namespace units {
-
-namespace physical {
+namespace units::physical {
 
 template<typename Dim, template<typename...> typename DimTemplate>
 concept DimensionOf = Dimension<Dim> && is_derived_from_instantiation<Dim, DimTemplate>;
@@ -178,140 +176,138 @@ struct dim_permeability : derived_dimension<Child, U, exp<H, 1>, exp<L, -1>> {};
 template<typename Child, Unit U, DimensionOf<dim_energy> E, DimensionOf<dim_substance> M>
 struct dim_molar_energy : derived_dimension<Child, U, exp<E, 1>, exp<M, -1>> {};
 
-}  // namespace physical
+template<typename T>
+concept Length = QuantityOf<T, dim_length>;
 
 template<typename T>
-concept Length = physical::QuantityOf<T, physical::dim_length>;
+concept Mass = QuantityOf<T, dim_mass>;
 
 template<typename T>
-concept Mass = physical::QuantityOf<T, physical::dim_mass>;
+concept Time = QuantityOf<T, dim_time>;
 
 template<typename T>
-concept Time = physical::QuantityOf<T, physical::dim_time>;
+concept Current = QuantityOf<T, dim_electric_current>;
 
 template<typename T>
-concept Current = physical::QuantityOf<T, physical::dim_electric_current>;
+concept Temperature = QuantityOf<T, dim_thermodynamic_temperature>;
 
 template<typename T>
-concept Temperature = physical::QuantityOf<T, physical::dim_thermodynamic_temperature>;
+concept Substance = QuantityOf<T, dim_substance>;
 
 template<typename T>
-concept Substance = physical::QuantityOf<T, physical::dim_substance>;
+concept LuminousIntensity = QuantityOf<T, dim_luminous_intensity>;
 
 template<typename T>
-concept LuminousIntensity = physical::QuantityOf<T, physical::dim_luminous_intensity>;
+concept Frequency = QuantityOf<T, dim_frequency>;
 
 template<typename T>
-concept Frequency = physical::QuantityOf<T, physical::dim_frequency>;
+concept Area = QuantityOf<T, dim_area>;
 
 template<typename T>
-concept Area = physical::QuantityOf<T, physical::dim_area>;
+concept Volume = QuantityOf<T, dim_volume>;
 
 template<typename T>
-concept Volume = physical::QuantityOf<T, physical::dim_volume>;
+concept Velocity = QuantityOf<T, dim_velocity>;
 
 template<typename T>
-concept Velocity = physical::QuantityOf<T, physical::dim_velocity>;
+concept Acceleration = QuantityOf<T, dim_acceleration>;
 
 template<typename T>
-concept Acceleration = physical::QuantityOf<T, physical::dim_acceleration>;
+concept Force = QuantityOf<T, dim_force>;
 
 template<typename T>
-concept Force = physical::QuantityOf<T, physical::dim_force>;
+concept Momentum = QuantityOf<T, dim_momentum>;
 
 template<typename T>
-concept Momentum = physical::QuantityOf<T, physical::dim_momentum>;
+concept Energy = QuantityOf<T, dim_energy>;
 
 template<typename T>
-concept Energy = physical::QuantityOf<T, physical::dim_energy>;
+concept Density = QuantityOf<T, dim_density>;
 
 template<typename T>
-concept Density = physical::QuantityOf<T, physical::dim_density>;
+concept Power = QuantityOf<T, dim_power>;
 
 template<typename T>
-concept Power = physical::QuantityOf<T, physical::dim_power>;
+concept Voltage = QuantityOf<T, dim_voltage>;
 
 template<typename T>
-concept Voltage = physical::QuantityOf<T, physical::dim_voltage>;
+concept ElectricCharge = QuantityOf<T, dim_electric_charge>;
 
 template<typename T>
-concept ElectricCharge = physical::QuantityOf<T, physical::dim_electric_charge>;
+concept Capacitance = QuantityOf<T, dim_capacitance>;
 
 template<typename T>
-concept Capacitance = physical::QuantityOf<T, physical::dim_capacitance>;
+concept SurfaceTension = QuantityOf<T, dim_surface_tension>;
 
 template<typename T>
-concept SurfaceTension = physical::QuantityOf<T, physical::dim_surface_tension>;
+concept Pressure = QuantityOf<T, dim_pressure>;
 
 template<typename T>
-concept Pressure = physical::QuantityOf<T, physical::dim_pressure>;
+concept MagneticInduction = QuantityOf<T, dim_magnetic_induction>;
 
 template<typename T>
-concept MagneticInduction = physical::QuantityOf<T, physical::dim_magnetic_induction>;
+concept MagneticFlux = QuantityOf<T, dim_magnetic_flux>;
 
 template<typename T>
-concept MagneticFlux = physical::QuantityOf<T, physical::dim_magnetic_flux>;
+concept Inductance = QuantityOf<T, dim_inductance>;
 
 template<typename T>
-concept Inductance = physical::QuantityOf<T, physical::dim_inductance>;
-
-template<typename T>
-concept Conductance = physical::QuantityOf<T, physical::dim_conductance>;
+concept Conductance = QuantityOf<T, dim_conductance>;
 
 // TODO Add when downcasting issue is solved
 // template<typename T>
-// concept Radioactivity = physical::QuantityOf<T, physical::dim_radioactivity>;
+// concept Radioactivity = QuantityOf<T, dim_radioactivity>;
 
 template<typename T>
-concept CatalyticActivity = physical::QuantityOf<T, physical::dim_catalytic_activity>;
+concept CatalyticActivity = QuantityOf<T, dim_catalytic_activity>;
 
 template<typename T>
-concept AbsorbedDose = physical::QuantityOf<T, physical::dim_absorbed_dose>;
+concept AbsorbedDose = QuantityOf<T, dim_absorbed_dose>;
 
 template<typename T>
-concept CurrentDensity = physical::QuantityOf<T, physical::dim_current_density>;
+concept CurrentDensity = QuantityOf<T, dim_current_density>;
 
 template<typename T>
-concept Concentration = physical::QuantityOf<T, physical::dim_concentration>;
+concept Concentration = QuantityOf<T, dim_concentration>;
 
 template<typename T>
-concept Luminance = physical::QuantityOf<T, physical::dim_luminance>;
+concept Luminance = QuantityOf<T, dim_luminance>;
 
 template<typename T>
-concept DynamicViscosity = physical::QuantityOf<T, physical::dim_dynamic_viscosity>;
+concept DynamicViscosity = QuantityOf<T, dim_dynamic_viscosity>;
 
 template<typename T>
-concept HeatCapacity = physical::QuantityOf<T, physical::dim_heat_capacity>;
+concept HeatCapacity = QuantityOf<T, dim_heat_capacity>;
 
 template<typename T>
-concept SpecificHeatCapacity = physical::QuantityOf<T, physical::dim_specific_heat_capacity>;
+concept SpecificHeatCapacity = QuantityOf<T, dim_specific_heat_capacity>;
 
 template<typename T>
-concept MolarHeatCapacity = physical::QuantityOf<T, physical::dim_molar_heat_capacity>;
+concept MolarHeatCapacity = QuantityOf<T, dim_molar_heat_capacity>;
 
 template<typename T>
-concept ThermalConductivity = physical::QuantityOf<T, physical::dim_thermal_conductivity>;
+concept ThermalConductivity = QuantityOf<T, dim_thermal_conductivity>;
 
 // TODO Add when downcasting issue is solved
 // template<typename T>
-// concept EnergyDensity = physical::QuantityOf<T, physical::dim_energy_density>;
+// concept EnergyDensity = QuantityOf<T, dim_energy_density>;
 
 template<typename T>
-concept ElectricFieldStrength = physical::QuantityOf<T, physical::dim_electric_field_strength>;
+concept ElectricFieldStrength = QuantityOf<T, dim_electric_field_strength>;
 
 template<typename T>
-concept ChargeDensity = physical::QuantityOf<T, physical::dim_charge_density>;
+concept ChargeDensity = QuantityOf<T, dim_charge_density>;
 
 template<typename T>
-concept SurfaceChargeDensity = physical::QuantityOf<T, physical::dim_surface_charge_density>;
+concept SurfaceChargeDensity = QuantityOf<T, dim_surface_charge_density>;
 
 template<typename T>
-concept Permittivity = physical::QuantityOf<T, physical::dim_permittivity>;
+concept Permittivity = QuantityOf<T, dim_permittivity>;
 
 template<typename T>
-concept Permeability = physical::QuantityOf<T, physical::dim_permeability>;
+concept Permeability = QuantityOf<T, dim_permeability>;
 
 template<typename T>
-concept MolarEnergy = physical::QuantityOf<T, physical::dim_molar_energy>;
+concept MolarEnergy = QuantityOf<T, dim_molar_energy>;
 
-}  // namespace units
+}  // namespace units::physical
diff --git a/src/include/units/physical/iau/length.h b/src/include/units/physical/iau/length.h
index f3ca07fd..e7e58d11 100644
--- a/src/include/units/physical/iau/length.h
+++ b/src/include/units/physical/iau/length.h
@@ -25,7 +25,7 @@
 
 #include <units/physical/si/length.h>
 
-namespace units::iau {
+namespace units::physical::iau {
 
 // https://en.wikipedia.org/wiki/Light-year
 struct light_year : named_scaled_unit<light_year, "ly", no_prefix, ratio<9460730472580800>, si::metre> {};
@@ -49,4 +49,4 @@ constexpr auto operator"" q_angstrom(long double l) { return si::length<angstrom
 
 }  // namespace literals
 
-}  // namespace units::iau
+}  // namespace units::physical::iau
diff --git a/src/include/units/physical/imperial/length.h b/src/include/units/physical/imperial/length.h
index 3db68318..0d81aa8f 100644
--- a/src/include/units/physical/imperial/length.h
+++ b/src/include/units/physical/imperial/length.h
@@ -24,7 +24,7 @@
 
 #include <units/physical/international/length.h>
 
-namespace units::imperial {
+namespace units::physical::imperial {
 
 // https://en.wikipedia.org/wiki/Chain_(unit)
 struct chain : named_scaled_unit<chain, "ch", no_prefix, ratio<22, 1>, international::yard> {};
@@ -42,4 +42,4 @@ constexpr auto operator"" q_rd(long double l) { return si::length<rod, long doub
 
 }  // namespace literals
 
-}  // namespace units::imperial
+}  // namespace units::physical::imperial
diff --git a/src/include/units/physical/international/area.h b/src/include/units/physical/international/area.h
index dcc09320..65686c3d 100644
--- a/src/include/units/physical/international/area.h
+++ b/src/include/units/physical/international/area.h
@@ -25,7 +25,7 @@
 #include <units/physical/si/area.h>
 #include <units/physical/international/length.h>
 
-namespace units::international {
+namespace units::physical::international {
 
 struct square_foot : deduced_unit<square_foot, si::dim_area, international::foot> {};
 
@@ -37,4 +37,4 @@ constexpr auto operator"" q_ft2(long double l) { return si::area<square_foot, lo
 
 }  // namespace literals
 
-}  // namespace units::international
+}  // namespace units::physical::international
diff --git a/src/include/units/physical/international/length.h b/src/include/units/physical/international/length.h
index 3cd32e5e..3403e6e4 100644
--- a/src/include/units/physical/international/length.h
+++ b/src/include/units/physical/international/length.h
@@ -25,7 +25,7 @@
 
 #include <units/physical/si/length.h>
 
-namespace units::international {
+namespace units::physical::international {
 
 // international yard
 // https://en.wikipedia.org/wiki/International_yard_and_pound
@@ -94,4 +94,4 @@ constexpr auto operator"" q_mil(long double l) { return si::length<mil, long dou
 
 }  // namespace literals
 
-}  // namespace units::international
+}  // namespace units::physical::international
diff --git a/src/include/units/physical/international/velocity.h b/src/include/units/physical/international/velocity.h
index 0290d8f3..d45c47d4 100644
--- a/src/include/units/physical/international/velocity.h
+++ b/src/include/units/physical/international/velocity.h
@@ -25,7 +25,7 @@
 #include <units/physical/si/velocity.h>
 #include <units/physical/international/length.h>
 
-namespace units::international {
+namespace units::physical::international {
 
 struct mile_per_hour : deduced_unit<mile_per_hour, si::dim_velocity, international::mile, si::hour> {};
 
@@ -37,4 +37,4 @@ constexpr auto operator"" q_mi_per_h(long double l) { return si::velocity<mile_p
 
 }  // namespace literals
 
-}  // namespace units::international
+}  // namespace units::physical::international
diff --git a/src/include/units/physical/international/volume.h b/src/include/units/physical/international/volume.h
index 9d596600..82f753d6 100644
--- a/src/include/units/physical/international/volume.h
+++ b/src/include/units/physical/international/volume.h
@@ -25,7 +25,7 @@
 #include <units/physical/si/volume.h>
 #include <units/physical/international/length.h>
 
-namespace units::international {
+namespace units::physical::international {
 
 struct cubic_foot : deduced_unit<cubic_foot, si::dim_volume, international::foot> {};
 
@@ -37,4 +37,4 @@ constexpr auto operator"" q_ft3(long double l) { return si::volume<cubic_foot, l
 
 }  // namespace literals
 
-}  // namespace units::international
+}  // namespace units::physical::international
diff --git a/src/include/units/physical/natural/constants.h b/src/include/units/physical/natural/constants.h
index e8350da8..8f4cdfeb 100644
--- a/src/include/units/physical/natural/constants.h
+++ b/src/include/units/physical/natural/constants.h
@@ -24,9 +24,9 @@
 
 #include <units/physical/natural/dimensions.h>
 
-namespace units::natural {
+namespace units::physical::natural {
 
 template<Scalar Rep = double>
 inline constexpr auto speed_of_light = velocity<unitless, Rep>(1);
 
-}  // namespace units::natural
+}  // namespace units::physical::natural
diff --git a/src/include/units/physical/natural/dimensions.h b/src/include/units/physical/natural/dimensions.h
index bd570a61..b3976bfa 100644
--- a/src/include/units/physical/natural/dimensions.h
+++ b/src/include/units/physical/natural/dimensions.h
@@ -26,7 +26,7 @@
 #include <units/physical/natural/units.h>
 #include <units/quantity.h>
 
-namespace units::natural {
+namespace units::physical::natural {
 
 struct dim_length : physical::dim_length<inverted_gigaelectronvolt> {};
 template<Unit U, Scalar Rep = double>
@@ -63,4 +63,4 @@ using energy = quantity<dim_force, U, Rep>;
 // Typical UDLs will not work here as the same units are reused by many quantities.
 // Should we define some strange ones (i.e. q_mass_GeV)?
 
-}  // namespace units::natural
+}  // namespace units::physical::natural
diff --git a/src/include/units/physical/natural/units.h b/src/include/units/physical/natural/units.h
index 1277c9eb..7d826957 100644
--- a/src/include/units/physical/natural/units.h
+++ b/src/include/units/physical/natural/units.h
@@ -25,7 +25,7 @@
 #include <units/unit.h>
 #include <units/physical/si/prefixes.h>
 
-namespace units::natural {
+namespace units::physical::natural {
 
 struct unitless : named_unit<unitless, "", no_prefix> {};
 struct electronvolt : named_unit<electronvolt, "eV", si::prefix> {};
@@ -38,4 +38,4 @@ struct square_gigaelectronvolt : named_unit<square_gigaelectronvolt, {"GeV²", "
 // natural units as an isolated island with ev = 1 which simplifies all
 // the maths a lot?
 
-}  // namespace units::natural
+}  // namespace units::physical::natural
diff --git a/src/include/units/physical/si/absorbed_dose.h b/src/include/units/physical/si/absorbed_dose.h
index 73fe7d50..956c8609 100644
--- a/src/include/units/physical/si/absorbed_dose.h
+++ b/src/include/units/physical/si/absorbed_dose.h
@@ -28,7 +28,7 @@
 #include <units/physical/si/prefixes.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct gray : named_unit<gray, "Gy", prefix> {};
 struct yoctogray : prefixed_unit<yoctogray, yocto, gray> {};
@@ -145,4 +145,4 @@ constexpr auto operator"" q_YGy(long double l) { return absorbed_dose<yottagray,
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/acceleration.h b/src/include/units/physical/si/acceleration.h
index 084d0edb..c1a61242 100644
--- a/src/include/units/physical/si/acceleration.h
+++ b/src/include/units/physical/si/acceleration.h
@@ -26,7 +26,7 @@
 #include <units/physical/si/velocity.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct metre_per_second_sq : unit<metre_per_second_sq> {};
 struct dim_acceleration : physical::dim_acceleration<dim_acceleration, metre_per_second_sq, dim_length, dim_time> {};
@@ -42,4 +42,4 @@ constexpr auto operator"" q_m_per_s2(long double l) { return acceleration<metre_
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/area.h b/src/include/units/physical/si/area.h
index f7e5e9fe..34fb9f4d 100644
--- a/src/include/units/physical/si/area.h
+++ b/src/include/units/physical/si/area.h
@@ -26,7 +26,7 @@
 #include <units/physical/si/length.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct square_metre : unit<square_metre> {};
 struct dim_area : physical::dim_area<dim_area, square_metre, dim_length> {};
@@ -149,4 +149,4 @@ constexpr auto operator"" q_ha(long double l) { return area<hectare, long double
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/capacitance.h b/src/include/units/physical/si/capacitance.h
index f9772f57..640da58c 100644
--- a/src/include/units/physical/si/capacitance.h
+++ b/src/include/units/physical/si/capacitance.h
@@ -28,7 +28,7 @@
 #include <units/physical/si/prefixes.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct farad : named_unit<farad, "F", prefix> {};
 struct yoctofarad : prefixed_unit<yoctofarad, yocto, farad> {};
@@ -145,4 +145,4 @@ constexpr auto operator"" q_YF(long double l) { return capacitance<yottafarad, l
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/catalytic_activity.h b/src/include/units/physical/si/catalytic_activity.h
index e1910e16..7ae6c0ed 100644
--- a/src/include/units/physical/si/catalytic_activity.h
+++ b/src/include/units/physical/si/catalytic_activity.h
@@ -28,7 +28,7 @@
 #include <units/physical/si/prefixes.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct katal : named_unit<katal, "kat", prefix> {};
 struct yoctokatal : prefixed_unit<yoctokatal, yocto, katal> {};
@@ -151,5 +151,5 @@ constexpr auto operator"" q_U(long double l) { return catalytic_activity<enzyme_
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
 
diff --git a/src/include/units/physical/si/charge_density.h b/src/include/units/physical/si/charge_density.h
index db2f04fe..04ba4a45 100644
--- a/src/include/units/physical/si/charge_density.h
+++ b/src/include/units/physical/si/charge_density.h
@@ -28,7 +28,7 @@
 #include <units/physical/si/prefixes.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct coulomb_per_metre_cub : unit<coulomb_per_metre_cub> {};
 struct coulomb_per_metre_sq : unit<coulomb_per_metre_sq> {};
@@ -54,4 +54,4 @@ constexpr auto operator"" q_C_per_m2(long double l) { return surface_charge_dens
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/concentration.h b/src/include/units/physical/si/concentration.h
index f566b9f3..4c6c6220 100644
--- a/src/include/units/physical/si/concentration.h
+++ b/src/include/units/physical/si/concentration.h
@@ -27,7 +27,7 @@
 #include <units/physical/si/substance.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct mol_per_metre_cub : unit<mol_per_metre_cub> {};
 struct dim_concentration : physical::dim_concentration<dim_concentration, mol_per_metre_cub, dim_substance, dim_length> {};
@@ -43,5 +43,5 @@ constexpr auto operator"" q_mol_per_m3(long double l) { return concentration<mol
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
 
diff --git a/src/include/units/physical/si/conductance.h b/src/include/units/physical/si/conductance.h
index ecd21632..dd7b16b4 100644
--- a/src/include/units/physical/si/conductance.h
+++ b/src/include/units/physical/si/conductance.h
@@ -27,7 +27,7 @@
 #include <units/physical/si/prefixes.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct siemens : named_unit<siemens, "S", prefix> {};
 struct yoctosiemens : prefixed_unit<yoctosiemens, yocto, siemens> {};
@@ -124,5 +124,5 @@ constexpr auto operator"" q_YS(long double l) { return conductance<yottasiemens,
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
 
diff --git a/src/include/units/physical/si/constants.h b/src/include/units/physical/si/constants.h
index 25891a1f..720b87dd 100644
--- a/src/include/units/physical/si/constants.h
+++ b/src/include/units/physical/si/constants.h
@@ -30,7 +30,7 @@
 #include <units/physical/si/temperature.h>
 #include <units/physical/si/velocity.h>
 
-namespace units::si::si2019 {
+namespace units::physical::si::si2019 {
 
 template<Scalar Rep = double>
 inline constexpr auto planck_constant = energy<joule, Rep>(6.62607015e-34) * time<second, Rep>(1);
@@ -59,4 +59,4 @@ inline constexpr auto hyperfine_structure_transition_frequency = frequency<hertz
 template<Scalar Rep = double>
 inline constexpr auto standard_gravity = acceleration<metre_per_second_sq, Rep>(9.80665);
 
-}  // namespace units::si::si2019
+}  // namespace units::physical::si::si2019
diff --git a/src/include/units/physical/si/current.h b/src/include/units/physical/si/current.h
index b9b956c8..b4349006 100644
--- a/src/include/units/physical/si/current.h
+++ b/src/include/units/physical/si/current.h
@@ -26,7 +26,7 @@
 #include <units/physical/si/prefixes.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct ampere : named_unit<ampere, "A", prefix> {};
 struct yoctoampere : prefixed_unit<yoctoampere, yocto, ampere> {};
@@ -143,4 +143,4 @@ constexpr auto operator"" q_YA(long double l) { return current<yottaampere, long
 
 } // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/current_density.h b/src/include/units/physical/si/current_density.h
index 6d6c6086..157c5cec 100644
--- a/src/include/units/physical/si/current_density.h
+++ b/src/include/units/physical/si/current_density.h
@@ -28,7 +28,7 @@
 #include <units/physical/si/prefixes.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct ampere_per_metre_sq : unit<ampere_per_metre_sq> {};
 
@@ -44,4 +44,4 @@ constexpr auto operator"" q_A_per_m2(long double l) { return current_density<amp
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/density.h b/src/include/units/physical/si/density.h
index 8a72e910..79407611 100644
--- a/src/include/units/physical/si/density.h
+++ b/src/include/units/physical/si/density.h
@@ -28,7 +28,7 @@
 #include <units/physical/si/prefixes.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct kilogram_per_metre_cub : unit<kilogram_per_metre_cub> {};
 
@@ -44,4 +44,4 @@ constexpr auto operator"" q_kg_per_m3(long double l) { return density<kilogram_p
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/dynamic_viscosity.h b/src/include/units/physical/si/dynamic_viscosity.h
index e3b984f6..412b4fe3 100644
--- a/src/include/units/physical/si/dynamic_viscosity.h
+++ b/src/include/units/physical/si/dynamic_viscosity.h
@@ -27,7 +27,7 @@
 #include <units/physical/si/pressure.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct pascal_second : unit<pascal_second> {};
 struct dim_dynamic_viscosity : physical::dim_dynamic_viscosity<dim_dynamic_viscosity, pascal_second, dim_pressure, dim_time> {};
@@ -43,5 +43,5 @@ constexpr auto operator"" q_Pa_s(long double l) { return dynamic_viscosity<pasca
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
 
diff --git a/src/include/units/physical/si/electric_charge.h b/src/include/units/physical/si/electric_charge.h
index f3460d18..1bdc153b 100644
--- a/src/include/units/physical/si/electric_charge.h
+++ b/src/include/units/physical/si/electric_charge.h
@@ -27,7 +27,7 @@
 #include <units/physical/si/time.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct coulomb : named_unit<coulomb, "C", prefix> {};
 
@@ -44,4 +44,4 @@ constexpr auto operator"" q_C(long double l) { return electric_charge<coulomb, l
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/electric_field_strength.h b/src/include/units/physical/si/electric_field_strength.h
index 1e42301e..dead3967 100644
--- a/src/include/units/physical/si/electric_field_strength.h
+++ b/src/include/units/physical/si/electric_field_strength.h
@@ -26,7 +26,7 @@
 #include <units/physical/si/voltage.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct volt_per_metre : unit<volt_per_metre> {};
 struct dim_electric_field_strength : physical::dim_electric_field_strength<dim_electric_field_strength, volt_per_metre, dim_voltage, dim_length> {};
@@ -42,4 +42,4 @@ constexpr auto operator"" q_V_per_m(long double l) { return electric_field_stren
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/energy.h b/src/include/units/physical/si/energy.h
index 48b4d3a2..54848946 100644
--- a/src/include/units/physical/si/energy.h
+++ b/src/include/units/physical/si/energy.h
@@ -27,7 +27,7 @@
 #include <units/physical/si/prefixes.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct joule : named_unit<joule, "J", prefix> {};
 struct yoctojoule : prefixed_unit<yoctojoule, yocto, joule> {};
@@ -135,4 +135,4 @@ constexpr auto operator"" q_GeV(long double l) { return energy<gigaelectronvolt,
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/force.h b/src/include/units/physical/si/force.h
index 7c8562c3..835b39a9 100644
--- a/src/include/units/physical/si/force.h
+++ b/src/include/units/physical/si/force.h
@@ -28,7 +28,7 @@
 #include <units/physical/si/prefixes.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct newton : named_unit<newton, "N", prefix> {};
 struct yoctonewton : prefixed_unit<yoctonewton, yocto, newton> {};
@@ -145,4 +145,4 @@ constexpr auto operator"" q_YN(long double l) { return force<yottanewton, long d
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/frequency.h b/src/include/units/physical/si/frequency.h
index 70e08160..c75d8213 100644
--- a/src/include/units/physical/si/frequency.h
+++ b/src/include/units/physical/si/frequency.h
@@ -26,7 +26,7 @@
 #include <units/physical/si/time.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct hertz : named_unit<hertz, "Hz", prefix> {};
 struct yoctohertz : prefixed_unit<yoctohertz, yocto, hertz> {};
@@ -123,4 +123,4 @@ constexpr auto operator"" q_YHz(long double l) { return frequency<yottahertz, lo
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/heat_capacity.h b/src/include/units/physical/si/heat_capacity.h
index d9ad90c7..87897314 100644
--- a/src/include/units/physical/si/heat_capacity.h
+++ b/src/include/units/physical/si/heat_capacity.h
@@ -29,7 +29,7 @@
 #include <units/physical/si/substance.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct joule_per_kelvin : unit<joule_per_kelvin> {};
 struct joule_per_kilogram_kelvin : unit<joule_per_kilogram_kelvin> {};
@@ -64,5 +64,5 @@ constexpr auto operator"" q_J_per_mol_K(long double l) { return molar_heat_capac
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
 
diff --git a/src/include/units/physical/si/inductance.h b/src/include/units/physical/si/inductance.h
index 6eeef43e..105a17cb 100644
--- a/src/include/units/physical/si/inductance.h
+++ b/src/include/units/physical/si/inductance.h
@@ -27,7 +27,7 @@
 #include <units/physical/si/current.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct henry : named_unit<henry, "H", prefix> {};
 
@@ -125,4 +125,4 @@ constexpr auto operator"" q_YH(long double l) { return inductance<yottahenry, lo
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/length.h b/src/include/units/physical/si/length.h
index 11f8a26b..a5c51a67 100644
--- a/src/include/units/physical/si/length.h
+++ b/src/include/units/physical/si/length.h
@@ -26,7 +26,7 @@
 #include <units/physical/si/prefixes.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct metre : named_unit<metre, "m", prefix> {};
 struct yoctometre : prefixed_unit<yoctometre, yocto, metre> {};
@@ -149,4 +149,4 @@ constexpr auto operator"" q_au(long double l) { return length<astronomical_unit,
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/luminance.h b/src/include/units/physical/si/luminance.h
index 1ef2a955..2af847c0 100644
--- a/src/include/units/physical/si/luminance.h
+++ b/src/include/units/physical/si/luminance.h
@@ -27,7 +27,7 @@
 #include <units/physical/si/luminous_intensity.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct candela_per_metre_sq : unit<candela_per_metre_sq> {};
 struct dim_luminance : physical::dim_luminance<dim_luminance, candela_per_metre_sq, dim_luminous_intensity, dim_length> {};
@@ -43,5 +43,5 @@ constexpr auto operator"" q_cd_per_m2(long double l) { return luminance<candela_
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
 
diff --git a/src/include/units/physical/si/luminous_intensity.h b/src/include/units/physical/si/luminous_intensity.h
index 2c546b66..0cca6300 100644
--- a/src/include/units/physical/si/luminous_intensity.h
+++ b/src/include/units/physical/si/luminous_intensity.h
@@ -26,7 +26,7 @@
 #include <units/physical/si/prefixes.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct candela : named_unit<candela, "cd", prefix> {};
 struct yoctocandela : prefixed_unit<yoctocandela, yocto, candela> {};
@@ -143,4 +143,4 @@ constexpr auto operator"" q_Ycd(long double l) { return luminous_intensity<yotta
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/magnetic_flux.h b/src/include/units/physical/si/magnetic_flux.h
index b857d9e0..433db1ee 100644
--- a/src/include/units/physical/si/magnetic_flux.h
+++ b/src/include/units/physical/si/magnetic_flux.h
@@ -27,7 +27,7 @@
 #include <units/physical/si/area.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct weber : named_unit<weber, "Wb", prefix> {};
 
@@ -125,4 +125,4 @@ constexpr auto operator"" q_YWb(long double l) { return magnetic_flux<yottaweber
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/magnetic_induction.h b/src/include/units/physical/si/magnetic_induction.h
index e3511487..fc20a69b 100644
--- a/src/include/units/physical/si/magnetic_induction.h
+++ b/src/include/units/physical/si/magnetic_induction.h
@@ -29,7 +29,7 @@
 #include <units/physical/si/prefixes.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct tesla : named_unit<tesla, "T", prefix> {};
 
diff --git a/src/include/units/physical/si/mass.h b/src/include/units/physical/si/mass.h
index f72aef2a..7a5e3337 100644
--- a/src/include/units/physical/si/mass.h
+++ b/src/include/units/physical/si/mass.h
@@ -26,7 +26,7 @@
 #include <units/physical/si/prefixes.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct gram : named_unit<gram, "g", prefix> {};
 struct yoctogram : prefixed_unit<yoctogram, yocto, gram> {};
@@ -255,4 +255,4 @@ constexpr auto operator"" q_Da(long double l) { return mass<dalton, long double>
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/molar_energy.h b/src/include/units/physical/si/molar_energy.h
index 6e3cb432..6d8b5d64 100644
--- a/src/include/units/physical/si/molar_energy.h
+++ b/src/include/units/physical/si/molar_energy.h
@@ -28,7 +28,7 @@
 #include <units/physical/si/prefixes.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct joule_per_mole : unit<joule_per_mole> {};
 
@@ -45,4 +45,4 @@ constexpr auto operator"" q_J_per_mol(long double l) { return molar_energy<joule
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/momentum.h b/src/include/units/physical/si/momentum.h
index cc1d128c..eb6cfde0 100644
--- a/src/include/units/physical/si/momentum.h
+++ b/src/include/units/physical/si/momentum.h
@@ -27,7 +27,7 @@
 #include <units/physical/si/velocity.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct kilogram_metre_per_second : unit<kilogram_metre_per_second> {};
 struct dim_momentum : physical::dim_momentum<dim_momentum, kilogram_metre_per_second, dim_mass, dim_velocity> {};
@@ -43,4 +43,4 @@ constexpr auto operator"" q_kg_m_per_s(long double l) { return momentum<kilogram
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/permeability.h b/src/include/units/physical/si/permeability.h
index e71c68ae..752acf04 100644
--- a/src/include/units/physical/si/permeability.h
+++ b/src/include/units/physical/si/permeability.h
@@ -27,7 +27,7 @@
 #include <units/physical/si/prefixes.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct henry_per_metre : unit<henry_per_metre> {};
 
@@ -44,5 +44,5 @@ constexpr auto operator"" q_H_per_m(long double l) { return permeability<henry_p
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
 
diff --git a/src/include/units/physical/si/permittivity.h b/src/include/units/physical/si/permittivity.h
index fed3396b..2f31fc26 100644
--- a/src/include/units/physical/si/permittivity.h
+++ b/src/include/units/physical/si/permittivity.h
@@ -27,7 +27,7 @@
 #include <units/physical/si/prefixes.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct farad_per_metre : unit<farad_per_metre> {};
 
@@ -44,5 +44,5 @@ constexpr auto operator"" q_F_per_m(long double l) { return permittivity<farad_p
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
 
diff --git a/src/include/units/physical/si/power.h b/src/include/units/physical/si/power.h
index e4846724..62c95239 100644
--- a/src/include/units/physical/si/power.h
+++ b/src/include/units/physical/si/power.h
@@ -27,7 +27,7 @@
 #include <units/physical/si/prefixes.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct watt : named_unit<watt, "W", prefix> {};
 struct yoctowatt : prefixed_unit<yoctowatt, yocto, watt> {};
@@ -124,4 +124,4 @@ constexpr auto operator"" q_YW(long double l) { return power<yottawatt, long dou
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/prefixes.h b/src/include/units/physical/si/prefixes.h
index 449b5754..fd0c5338 100644
--- a/src/include/units/physical/si/prefixes.h
+++ b/src/include/units/physical/si/prefixes.h
@@ -24,7 +24,7 @@
 
 #include <units/prefix.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct prefix : prefix_family {};
 
@@ -51,4 +51,4 @@ struct zetta  : units::prefix<zetta,  prefix, "Z",             ratio<1, 1,  21>>
 struct yotta  : units::prefix<yotta,  prefix, "Y",             ratio<1, 1,  24>> {};
 // clang-format on
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/pressure.h b/src/include/units/physical/si/pressure.h
index dd6f987c..6f68ce52 100644
--- a/src/include/units/physical/si/pressure.h
+++ b/src/include/units/physical/si/pressure.h
@@ -28,7 +28,7 @@
 #include <units/physical/si/prefixes.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct pascal : named_unit<pascal, "Pa", prefix> {};
 struct yoctopascal : prefixed_unit<yoctopascal, yocto, pascal> {};
@@ -145,4 +145,4 @@ constexpr auto operator"" q_YPa(long double l) { return pressure<yottapascal, lo
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/resistance.h b/src/include/units/physical/si/resistance.h
index 59b1d1a1..f096b0fe 100644
--- a/src/include/units/physical/si/resistance.h
+++ b/src/include/units/physical/si/resistance.h
@@ -28,7 +28,7 @@
 #include <units/physical/si/prefixes.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct ohm : named_unit<ohm, {"Ω", "ohm"}, prefix> {};
 struct yoctoohm : prefixed_unit<yoctoohm, yocto, ohm> {};
@@ -125,4 +125,4 @@ constexpr auto operator"" q_YR(long double l) { return resistance<yottaohm, long
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/substance.h b/src/include/units/physical/si/substance.h
index 2fc76f5f..e3d0348e 100644
--- a/src/include/units/physical/si/substance.h
+++ b/src/include/units/physical/si/substance.h
@@ -26,7 +26,7 @@
 #include <units/physical/si/prefixes.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct mole : named_unit<metre, "mol", prefix> {};
 
@@ -43,4 +43,4 @@ constexpr auto operator"" q_mol(long double l) { return substance<mole, long dou
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/surface_tension.h b/src/include/units/physical/si/surface_tension.h
index d63ffc9b..77befa13 100644
--- a/src/include/units/physical/si/surface_tension.h
+++ b/src/include/units/physical/si/surface_tension.h
@@ -26,7 +26,7 @@
 #include <units/physical/si/force.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct newton_per_metre : unit<newton_per_metre> {};
 
@@ -43,4 +43,4 @@ inline namespace literals {
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/temperature.h b/src/include/units/physical/si/temperature.h
index 79ee6398..cf016ebc 100644
--- a/src/include/units/physical/si/temperature.h
+++ b/src/include/units/physical/si/temperature.h
@@ -25,7 +25,7 @@
 #include <units/physical/dimensions.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct kelvin : named_unit<kelvin, "K", no_prefix> {};
 
@@ -42,4 +42,4 @@ constexpr auto operator"" q_K(long double l) { return temperature<kelvin, long d
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/thermal_conductivity.h b/src/include/units/physical/si/thermal_conductivity.h
index 6ff60219..b74f0b87 100644
--- a/src/include/units/physical/si/thermal_conductivity.h
+++ b/src/include/units/physical/si/thermal_conductivity.h
@@ -27,7 +27,7 @@
 #include <units/physical/si/temperature.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct watt_per_metre_kelvin : unit<watt_per_metre_kelvin> {};
 
@@ -44,4 +44,4 @@ constexpr auto operator"" q_W_per_m_K(long double l) { return thermal_conductivi
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/time.h b/src/include/units/physical/si/time.h
index 16409534..b3778bd8 100644
--- a/src/include/units/physical/si/time.h
+++ b/src/include/units/physical/si/time.h
@@ -26,7 +26,7 @@
 #include <units/physical/si/prefixes.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct second : named_unit<second, "s", prefix> {};
 struct yoctosecond : prefixed_unit<yoctosecond, yocto, second> {};
@@ -98,4 +98,4 @@ constexpr auto operator"" q_d(long double l) { return time<day, long double>(l);
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/velocity.h b/src/include/units/physical/si/velocity.h
index 7507da44..8a33fcde 100644
--- a/src/include/units/physical/si/velocity.h
+++ b/src/include/units/physical/si/velocity.h
@@ -27,7 +27,7 @@
 #include <units/physical/si/time.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct metre_per_second : unit<metre_per_second> {};
 struct dim_velocity : physical::dim_velocity<dim_velocity, metre_per_second, dim_length, dim_time> {};
@@ -49,4 +49,4 @@ constexpr auto operator"" q_km_per_h(long double l) { return velocity<kilometre_
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/voltage.h b/src/include/units/physical/si/voltage.h
index 51b46cb9..b974ddfd 100644
--- a/src/include/units/physical/si/voltage.h
+++ b/src/include/units/physical/si/voltage.h
@@ -28,7 +28,7 @@
 #include <units/physical/si/prefixes.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct volt : named_unit<volt, "V", prefix> {};
 struct yoctovolt : prefixed_unit<yoctovolt, yocto, volt> {};
@@ -145,4 +145,4 @@ constexpr auto operator"" q_YV(long double l) { return voltage<yottavolt, long d
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/si/volume.h b/src/include/units/physical/si/volume.h
index a2f34826..ad5affc8 100644
--- a/src/include/units/physical/si/volume.h
+++ b/src/include/units/physical/si/volume.h
@@ -26,7 +26,7 @@
 #include <units/physical/si/length.h>
 #include <units/quantity.h>
 
-namespace units::si {
+namespace units::physical::si {
 
 struct cubic_metre : unit<cubic_metre> {};
 struct dim_volume : physical::dim_volume<dim_volume, cubic_metre, dim_length> {};
@@ -249,4 +249,4 @@ constexpr auto operator"" q_Yl(long double l) { return volume<yottalitre, long d
 
 }  // namespace literals
 
-}  // namespace units::si
+}  // namespace units::physical::si
diff --git a/src/include/units/physical/typographic/length.h b/src/include/units/physical/typographic/length.h
index ab58b2ff..d1445d1c 100644
--- a/src/include/units/physical/typographic/length.h
+++ b/src/include/units/physical/typographic/length.h
@@ -25,7 +25,7 @@
 
 #include <units/physical/si/length.h>
 
-namespace units::typographic {
+namespace units::physical::typographic {
 
 // TODO Conflicts with (https://en.wikipedia.org/wiki/Pica_(typography)), verify correctness of below conversion factors and provide hyperlinks to definitions
 struct pica_comp : named_scaled_unit<pica_comp, "pica(comp)", no_prefix, ratio<4233333, 1000000, -3>, si::metre> {};
@@ -53,4 +53,4 @@ constexpr auto operator"" q_point_prn(long double l) { return si::length<point_p
 
 }  // namespace literals
 
-}  // namespace units::typographic
+}  // namespace units::physical::typographic
diff --git a/src/include/units/physical/us/length.h b/src/include/units/physical/us/length.h
index 6f1abca0..13aa1cda 100644
--- a/src/include/units/physical/us/length.h
+++ b/src/include/units/physical/us/length.h
@@ -24,7 +24,7 @@
 
 #include <units/physical/si/length.h>
 
-namespace units::us {
+namespace units::physical::us {
 
 // https://en.wikipedia.org/wiki/Foot_(unit)#US_survey_foot
 // https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors#B6
@@ -40,17 +40,17 @@ struct mile : named_scaled_unit<mile, "mi(us)", no_prefix, ratio<5280>, us::foot
 inline namespace literals {
 
 // ft
-constexpr auto operator"" q_ft_us(unsigned long long l) { return si::length<units::us::foot, std::int64_t>(l); }
-constexpr auto operator"" q_ft_us(long double l) { return si::length<units::us::foot, long double>(l); }
+constexpr auto operator"" q_ft_us(unsigned long long l) { return si::length<units::physical::us::foot, std::int64_t>(l); }
+constexpr auto operator"" q_ft_us(long double l) { return si::length<units::physical::us::foot, long double>(l); }
 
 // fathom
-constexpr auto operator"" q_fathom_us(unsigned long long l) { return si::length<units::us::fathom, std::int64_t>(l); }
-constexpr auto operator"" q_fathom_us(long double l) { return si::length<units::us::fathom, long double>(l); }
+constexpr auto operator"" q_fathom_us(unsigned long long l) { return si::length<units::physical::us::fathom, std::int64_t>(l); }
+constexpr auto operator"" q_fathom_us(long double l) { return si::length<units::physical::us::fathom, long double>(l); }
 
 // ft
-constexpr auto operator"" q_mi_us(unsigned long long l) { return si::length<units::us::mile, std::int64_t>(l); }
-constexpr auto operator"" q_mi_us(long double l) { return si::length<units::us::mile, long double>(l); }
+constexpr auto operator"" q_mi_us(unsigned long long l) { return si::length<units::physical::us::mile, std::int64_t>(l); }
+constexpr auto operator"" q_mi_us(long double l) { return si::length<units::physical::us::mile, long double>(l); }
 
 }  // namespace literals
 
-}  // namespace units::us
+}  // namespace units::physical::us
diff --git a/src/include/units/quantity_cast.h b/src/include/units/quantity_cast.h
index e160494d..7235ebb6 100644
--- a/src/include/units/quantity_cast.h
+++ b/src/include/units/quantity_cast.h
@@ -320,7 +320,7 @@ struct cast_ratio<FromD, FromU, ToD, ToU> {
  *
  * This cast gets the target quantity type to cast to. For example:
  *
- * auto q1 = units::quantity_cast<units::si::time<units::si::second>>(1q_ms);
+ * auto q1 = units::quantity_cast<units::physical::si::time<units::physical::si::second>>(1q_ms);
  *
  * @tparam To a target quantity type to cast to
  */
@@ -344,7 +344,7 @@ template<Quantity To, typename D, typename U, typename Rep>
  *
  * This cast gets only the target dimension to cast to. For example:
  *
- * auto q1 = units::quantity_cast<units::si::acceleration>(200q_Gal);
+ * auto q1 = units::quantity_cast<units::physical::si::acceleration>(200q_Gal);
  *
  * @tparam ToD a dimension type to use for a target quantity
  */
@@ -363,7 +363,7 @@ template<Dimension ToD, typename D, typename U, typename Rep>
  *
  * This cast gets only the target unit to cast to. For example:
  *
- * auto q1 = units::quantity_cast<units::si::second>(1q_ms);
+ * auto q1 = units::quantity_cast<units::physical::si::second>(1q_ms);
  *
  * @tparam ToU a unit type to use for a target quantity
  */
diff --git a/test/unit_test/runtime/fmt_test.cpp b/test/unit_test/runtime/fmt_test.cpp
index 4d306f5e..e08e623c 100644
--- a/test/unit_test/runtime/fmt_test.cpp
+++ b/test/unit_test/runtime/fmt_test.cpp
@@ -37,7 +37,8 @@
 #include <sstream>
 
 using namespace units;
-using namespace units::si;
+using namespace units::physical;
+using namespace units::physical::si;
 using namespace Catch::Matchers;
 
 TEST_CASE("operator<< on a quantity", "[text][ostream][fmt]")
@@ -290,7 +291,7 @@ TEST_CASE("operator<< on a quantity", "[text][ostream][fmt]")
 
       SECTION("surface tension")
       {
-        struct newton_per_centimetre : deduced_unit<newton_per_centimetre, dim_surface_tension, newton, centimetre> {};
+        struct newton_per_centimetre : deduced_unit<newton_per_centimetre, si::dim_surface_tension, newton, centimetre> {};
         const surface_tension<newton_per_centimetre> q(123);
         os << q;
 
diff --git a/test/unit_test/runtime/fmt_units_test.cpp b/test/unit_test/runtime/fmt_units_test.cpp
index 474ecff3..905bb3b2 100644
--- a/test/unit_test/runtime/fmt_units_test.cpp
+++ b/test/unit_test/runtime/fmt_units_test.cpp
@@ -32,12 +32,12 @@
 #include "units/format.h"
 #include <catch2/catch.hpp>
 
-using namespace units::si;
-using namespace units::international;
-using namespace units::us;
-using namespace units::iau;
-using namespace units::imperial;
-using namespace units::typographic;
+using namespace units::physical::si;
+using namespace units::physical::international;
+using namespace units::physical::us;
+using namespace units::physical::iau;
+using namespace units::physical::imperial;
+using namespace units::physical::typographic;
 
 TEST_CASE("fmt::format on synthesized unit symbols", "[text][fmt]")
 {
diff --git a/test/unit_test/runtime/math_test.cpp b/test/unit_test/runtime/math_test.cpp
index 617c1e5f..fc33ad3e 100644
--- a/test/unit_test/runtime/math_test.cpp
+++ b/test/unit_test/runtime/math_test.cpp
@@ -26,7 +26,7 @@
 #include <catch2/catch.hpp>
 
 using namespace units;
-using namespace units::si;
+using namespace units::physical::si;
 
 // classical
 
diff --git a/test/unit_test/static/cgs_test.cpp b/test/unit_test/static/cgs_test.cpp
index ac33c0f3..30140e48 100644
--- a/test/unit_test/static/cgs_test.cpp
+++ b/test/unit_test/static/cgs_test.cpp
@@ -34,7 +34,7 @@
 namespace {
 
 using namespace units;
-using namespace units::cgs;
+using namespace units::physical::cgs;
 
 /* ************** BASE DIMENSIONS **************** */
 
diff --git a/test/unit_test/static/custom_rep_min_req_test.cpp b/test/unit_test/static/custom_rep_min_req_test.cpp
index 477e07e5..acb74a08 100644
--- a/test/unit_test/static/custom_rep_min_req_test.cpp
+++ b/test/unit_test/static/custom_rep_min_req_test.cpp
@@ -165,7 +165,7 @@ struct quantity_values<impl<T>> {
 
 namespace {
 
-using namespace units::si;
+using namespace units::physical::si;
 
 // constructors
 
diff --git a/test/unit_test/static/custom_unit_test.cpp b/test/unit_test/static/custom_unit_test.cpp
index 08921a8c..b2e4aff1 100644
--- a/test/unit_test/static/custom_unit_test.cpp
+++ b/test/unit_test/static/custom_unit_test.cpp
@@ -29,7 +29,7 @@
 namespace {
 
 using namespace units;
-using namespace units::si;
+using namespace units::physical::si;
 
 // power spectral density
 struct sq_volt_per_hertz : unit<sq_volt_per_hertz> {};
@@ -60,8 +60,8 @@ static_assert(std::is_same_v<decltype(sqrt(power_spectral_density<sq_volt_per_he
 namespace {
 
 struct kilogram_per_second : unit<kilogram_per_second> {};
-struct dim_mass_rate : derived_dimension<dim_mass_rate, kilogram_per_second, units::exp<si::dim_mass, 1>, units::exp<si::dim_time, -1>> {};
-struct kilogram_per_hour : deduced_unit<kilogram_per_hour, dim_mass_rate, si::kilogram, si::hour> {};
+struct dim_mass_rate : derived_dimension<dim_mass_rate, kilogram_per_second, units::exp<dim_mass, 1>, units::exp<dim_time, -1>> {};
+struct kilogram_per_hour : deduced_unit<kilogram_per_hour, dim_mass_rate, kilogram, hour> {};
 constexpr auto a = 1q_kg / 1q_h;
 static_assert(std::is_same_v<decltype(a)::unit, kilogram_per_hour>);
 
diff --git a/test/unit_test/static/dimensions_concepts_test.cpp b/test/unit_test/static/dimensions_concepts_test.cpp
index c70dcbaa..88689de0 100644
--- a/test/unit_test/static/dimensions_concepts_test.cpp
+++ b/test/unit_test/static/dimensions_concepts_test.cpp
@@ -25,7 +25,7 @@
 
 namespace {
 
-using namespace units;
+using namespace units::physical;
 
 static_assert(Length<si::length<si::metre>>);
 static_assert(!Length<si::time<si::second>>);
diff --git a/test/unit_test/static/math_test.cpp b/test/unit_test/static/math_test.cpp
index fa4a3b2e..ab4c04fa 100644
--- a/test/unit_test/static/math_test.cpp
+++ b/test/unit_test/static/math_test.cpp
@@ -27,8 +27,8 @@
 
 namespace {
 
-  using namespace units::si::literals;
-  using namespace units::international::literals;
+  using namespace units::physical::si::literals;
+  using namespace units::physical::international::literals;
 
   static_assert(std::is_same_v<decltype(pow<0>(2q_m)), std::int64_t>);
   static_assert(std::is_same_v<decltype(pow<1>(2q_m)), decltype(2q_m)>);
diff --git a/test/unit_test/static/quantity_test.cpp b/test/unit_test/static/quantity_test.cpp
index d7c5c687..a5770f2a 100644
--- a/test/unit_test/static/quantity_test.cpp
+++ b/test/unit_test/static/quantity_test.cpp
@@ -31,7 +31,7 @@
 namespace {
 
 using namespace units;
-using namespace units::si;
+using namespace units::physical::si;
 
 // class invariants
 
@@ -135,24 +135,24 @@ static_assert(
 static_assert(std::is_same_v<decltype(length<metre, int>() * 1.0), length<metre, double>>);
 static_assert(std::is_same_v<decltype(1.0 * length<metre, int>()), length<metre, double>>);
 static_assert(
-    std::is_same_v<decltype(velocity<metre_per_second, int>() * si::time<second, int>()), length<metre, int>>);
+    std::is_same_v<decltype(velocity<metre_per_second, int>() * physical::si::time<second, int>()), length<metre, int>>);
 static_assert(
-    std::is_same_v<decltype(velocity<metre_per_second, int>() * si::time<hour, int>()), length<scaled_unit<ratio<36, 1, 2>, metre>, int>>);
-static_assert(std::is_same_v<decltype(length<metre>() * si::time<minute>()),
+    std::is_same_v<decltype(velocity<metre_per_second, int>() * physical::si::time<hour, int>()), length<scaled_unit<ratio<36, 1, 2>, metre>, int>>);
+static_assert(std::is_same_v<decltype(length<metre>() * physical::si::time<minute>()),
               quantity<unknown_dimension<units::exp<dim_length, 1>, units::exp<dim_time, 1>>, scaled_unit<ratio<6, 1, 1>, unknown_coherent_unit>>>);
-static_assert(std::is_same_v<decltype(1 / si::time<second, int>()), frequency<hertz, int>>);
-static_assert(std::is_same_v<decltype(1 / si::time<minute, int>()), frequency<scaled_unit<ratio<1, 6, -1>, hertz>, int>>);
-static_assert(std::is_same_v<decltype(1 / frequency<hertz, int>()), si::time<second, int>>);
+static_assert(std::is_same_v<decltype(1 / physical::si::time<second, int>()), frequency<hertz, int>>);
+static_assert(std::is_same_v<decltype(1 / physical::si::time<minute, int>()), frequency<scaled_unit<ratio<1, 6, -1>, hertz>, int>>);
+static_assert(std::is_same_v<decltype(1 / frequency<hertz, int>()), physical::si::time<second, int>>);
 static_assert(std::is_same_v<decltype(1 / length<kilometre>()),
               quantity<unknown_dimension<units::exp<dim_length, -1>>, scaled_unit<ratio<1, 1, -3>, unknown_coherent_unit>>>);
 static_assert(std::is_same_v<decltype(length<metre, int>() / 1.0), length<metre, double>>);
 static_assert(std::is_same_v<decltype(length<metre, int>() / length<metre, double>()), double>);
 static_assert(std::is_same_v<decltype(length<kilometre, int>() / length<metre, double>()), double>);
 static_assert(
-    std::is_same_v<decltype(length<metre, int>() / si::time<second, int>()), velocity<metre_per_second, int>>);
+    std::is_same_v<decltype(length<metre, int>() / physical::si::time<second, int>()), velocity<metre_per_second, int>>);
 static_assert(
-    std::is_same_v<decltype(length<metre>() / si::time<minute>()), velocity<scaled_unit<ratio<1, 6, -1>, metre_per_second>>>);
-static_assert(std::is_same_v<decltype(si::time<minute>() / length<metre>()),
+    std::is_same_v<decltype(length<metre>() / physical::si::time<minute>()), velocity<scaled_unit<ratio<1, 6, -1>, metre_per_second>>>);
+static_assert(std::is_same_v<decltype(physical::si::time<minute>() / length<metre>()),
               quantity<unknown_dimension<units::exp<dim_length, -1>, units::exp<dim_time, 1>>, scaled_unit<ratio<6 ,1 , 1>, unknown_coherent_unit>>>);
 static_assert(std::is_same_v<decltype(length<metre, int>() % short(1)), length<metre, int>>);
 static_assert(std::is_same_v<decltype(length<metre, int>() % length<metre, short>(1)), length<metre, int>>);
diff --git a/test/unit_test/static/si_cgs_test.cpp b/test/unit_test/static/si_cgs_test.cpp
index 8af2f99c..37abe968 100644
--- a/test/unit_test/static/si_cgs_test.cpp
+++ b/test/unit_test/static/si_cgs_test.cpp
@@ -44,7 +44,7 @@
 
 namespace {
 
-using namespace units;
+using namespace units::physical;
 
 static_assert(cgs::length<cgs::centimetre>(100) == si::length<si::metre>(1));
 static_assert(cgs::mass<cgs::gram>(1'000) == si::mass<si::kilogram>(1));
@@ -59,7 +59,7 @@ static_assert(cgs::pressure<cgs::barye>(10) == si::pressure<si::pascal>(1));
 
 namespace si_test {
 
-using namespace units::si::literals;
+using namespace units::physical::si::literals;
 
 static_assert(cgs::length<cgs::centimetre>(100) == 1q_m);
 static_assert(cgs::mass<cgs::gram>(1'000) == 1q_kg);
@@ -75,7 +75,7 @@ static_assert(cgs::pressure<cgs::barye>(10) == 1q_Pa);
 
 namespace cgs_test {
 
-using namespace units::cgs::literals;
+using namespace units::physical::cgs::literals;
 
 static_assert(100q_cm == si::length<si::metre>(1));
 static_assert(1'000q_g == si::mass<si::kilogram>(1));
@@ -91,8 +91,8 @@ static_assert(10q_Ba == si::pressure<si::pascal>(1));
 
 namespace both_test {
 
-using namespace units::si::literals;
-using namespace units::cgs::literals;
+using namespace units::physical::si::literals;
+using namespace units::physical::cgs::literals;
 
 // static_assert(100q_cm == 1q_m);   // ambiguous
 // static_assert(1'000q_g == 1q_kg); // ambiguous
diff --git a/test/unit_test/static/si_test.cpp b/test/unit_test/static/si_test.cpp
index f7dd6898..21f83b1c 100644
--- a/test/unit_test/static/si_test.cpp
+++ b/test/unit_test/static/si_test.cpp
@@ -26,7 +26,7 @@
 namespace {
 
 using namespace units;
-using namespace units::si;
+using namespace units::physical::si;
 
 /* ************** BASE DIMENSIONS **************** */
 
diff --git a/test/unit_test/static/unit_test.cpp b/test/unit_test/static/unit_test.cpp
index c0ecd215..6f6cc477 100644
--- a/test/unit_test/static/unit_test.cpp
+++ b/test/unit_test/static/unit_test.cpp
@@ -26,6 +26,7 @@
 namespace {
 
 using namespace units;
+using namespace units::physical;
 
 struct metre : named_unit<metre, "m", si::prefix> {};
 struct centimetre : prefixed_unit<centimetre, si::centi, metre> {};
diff --git a/test/unit_test/static/us_test.cpp b/test/unit_test/static/us_test.cpp
index 7b7d8f60..387dce1d 100644
--- a/test/unit_test/static/us_test.cpp
+++ b/test/unit_test/static/us_test.cpp
@@ -34,9 +34,9 @@
 namespace {
 
 using namespace units;
-using namespace units::si;
-using namespace units::us;
-using namespace units::international;
+using namespace units::physical::si;
+using namespace units::physical::us;
+using namespace units::physical::international;
 
 /* ************** BASE DIMENSIONS **************** */
 
diff --git a/test_package/test_package.cpp b/test_package/test_package.cpp
index 901168ff..efd7116f 100644
--- a/test_package/test_package.cpp
+++ b/test_package/test_package.cpp
@@ -30,6 +30,6 @@ constexpr units::Velocity AUTO avg_speed(units::Length AUTO d, units::Time AUTO
 
 int main()
 {
-  using namespace units::si::literals;
+  using namespace units::physical::si::literals;
   std::cout << "Average speed = " << avg_speed(240.q_km, 2q_h) << '\n';
 }