diff --git a/example/example.cpp b/example/example.cpp
index 2729bf33..6e89dda0 100644
--- a/example/example.cpp
+++ b/example/example.cpp
@@ -20,7 +20,7 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
-#include "units/si/velocity.h"
+#include <units/velocity.h>
 #include <iostream>
 
 namespace {
diff --git a/src/include/units/si/base_dimensions.h b/src/include/units/base_dimensions.h
similarity index 91%
rename from src/include/units/si/base_dimensions.h
rename to src/include/units/base_dimensions.h
index b939e86a..293bb227 100644
--- a/src/include/units/si/base_dimensions.h
+++ b/src/include/units/base_dimensions.h
@@ -22,7 +22,7 @@
 
 #pragma once
 
-#include "../dimension.h"
+#include <units/dimension.h>
 
 namespace units {
 
@@ -31,9 +31,9 @@ namespace units {
   struct base_dim_length : dim_id<0> {};
   struct base_dim_mass : dim_id<1> {};
   struct base_dim_time : dim_id<2> {};
-  struct base_dim_electric_current : dim_id<3> {};
+  struct base_dim_current : dim_id<3> {};
   struct base_dim_temperature : dim_id<4> {};
-  struct base_dim_amount_of_substance : dim_id<5> {};
+  struct base_dim_substance : dim_id<5> {};
   struct base_dim_luminous_intensity : dim_id<6> {};
 
 }  // namespace units
diff --git a/src/include/units/bits/type_list.h b/src/include/units/bits/type_list.h
index 2e25082c..5a493e8a 100644
--- a/src/include/units/bits/type_list.h
+++ b/src/include/units/bits/type_list.h
@@ -22,7 +22,7 @@
 
 #pragma once
 
-#include "tools.h"
+#include <units/bits/tools.h>
 #include <type_traits>
 
 namespace mp {
diff --git a/src/include/units/current.h b/src/include/units/current.h
new file mode 100644
index 00000000..06e918ab
--- /dev/null
+++ b/src/include/units/current.h
@@ -0,0 +1,50 @@
+// The MIT License (MIT)
+//
+// Copyright (c) 2018 Mateusz Pusz
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+#pragma once
+
+#include <units/base_dimensions.h>
+#include <units/quantity.h>
+
+namespace units {
+
+  struct dimension_current : make_dimension_t<exp<base_dim_current, 1>> {};
+  template<> struct upcasting_traits<upcast_from<dimension_current>> : upcast_to<dimension_current> {};
+
+  template<typename T>
+  concept bool Current = Quantity<T> && std::experimental::ranges::Same<typename T::dimension, dimension_current>;
+
+  template<Unit U = struct ampere, Number Rep = double>
+  using current = quantity<dimension_current, U, Rep>;
+
+  struct ampere : unit<dimension_current, std::ratio<1>> {};
+  template<> struct upcasting_traits<upcast_from<ampere>> : upcast_to<ampere> {};
+
+  inline namespace literals {
+
+    // A
+    constexpr auto operator""_A(unsigned long long l) { return current<ampere, std::int64_t>(l); }
+    constexpr auto operator""_A(long double l) { return current<ampere, long double>(l); }
+
+  }
+
+}  // namespace units
diff --git a/src/include/units/dimension.h b/src/include/units/dimension.h
index 29791b49..1ce9956c 100644
--- a/src/include/units/dimension.h
+++ b/src/include/units/dimension.h
@@ -22,8 +22,8 @@
 
 #pragma once
 
-#include "bits/tools.h"
-#include "bits/type_list.h"
+#include <units/bits/tools.h>
+#include <units/bits/type_list.h>
 
 namespace units {
 
diff --git a/src/include/units/si/frequency.h b/src/include/units/frequency.h
similarity index 97%
rename from src/include/units/si/frequency.h
rename to src/include/units/frequency.h
index 319a0a05..c2e8b37e 100644
--- a/src/include/units/si/frequency.h
+++ b/src/include/units/frequency.h
@@ -22,14 +22,20 @@
 
 #pragma once
 
-#include "base_dimensions.h"
-#include "time.h"
+#include <units/base_dimensions.h>
+#include <units/time.h>
 
 namespace units {
 
   struct dimension_frequency : make_dimension_t<exp<base_dim_time, -1>> {};
   template<> struct upcasting_traits<upcast_from<dimension_frequency>> : upcast_to<dimension_frequency> {};
 
+  template<typename T>
+  concept bool Frequency =  Quantity<T> && std::experimental::ranges::Same<typename T::dimension, dimension_frequency>;
+
+  template<Unit U = struct hertz, Number Rep = double>
+  using frequency = quantity<dimension_frequency, U, Rep>;
+
   struct millihertz : unit<dimension_frequency, std::milli> {};
   template<> struct upcasting_traits<upcast_from<millihertz>> : upcast_to<millihertz> {};
 
@@ -48,12 +54,6 @@ namespace units {
   struct terahertz : unit<dimension_frequency, std::tera> {};
   template<> struct upcasting_traits<upcast_from<terahertz>> : upcast_to<terahertz> {};
 
-  template<Unit U = hertz, Number Rep = double>
-  using frequency = quantity<dimension_frequency, U, Rep>;
-
-  template<typename T>
-  concept bool Frequency =  Quantity<T> && std::experimental::ranges::Same<typename T::dimension, dimension_frequency>;
-
   inline namespace literals {
 
     // mHz
diff --git a/src/include/units/si/length.h b/src/include/units/length.h
similarity index 96%
rename from src/include/units/si/length.h
rename to src/include/units/length.h
index 48766f21..be662b5c 100644
--- a/src/include/units/si/length.h
+++ b/src/include/units/length.h
@@ -22,15 +22,20 @@
 
 #pragma once
 
-#include "base_dimensions.h"
-#include "../quantity.h"
+#include <units/base_dimensions.h>
+#include <units/quantity.h>
 
 namespace units {
 
-  // dimension
   struct dimension_length : make_dimension_t<exp<base_dim_length, 1>> {};
   template<> struct upcasting_traits<upcast_from<dimension_length>> : upcast_to<dimension_length> {};
 
+  template<typename T>
+  concept bool Length = Quantity<T> && std::experimental::ranges::Same<typename T::dimension, dimension_length>;
+
+  template<Unit U = struct meter, Number Rep = double>
+  using length = quantity<dimension_length, U, Rep>;
+
   // SI units
   struct millimeter : unit<dimension_length, std::milli> {};
   template<> struct upcasting_traits<upcast_from<millimeter>> : upcast_to<millimeter> {};
@@ -44,26 +49,6 @@ namespace units {
   struct kilometer : unit<dimension_length, std::kilo> {};
   template<> struct upcasting_traits<upcast_from<kilometer>> : upcast_to<kilometer> {};
 
-  // US customary units
-  struct yard : unit<dimension_length, std::ratio<9'144, 10'000>> {};
-  template<> struct upcasting_traits<upcast_from<yard>> : upcast_to<yard> {};
-
-  struct foot : unit<dimension_length, std::ratio_multiply<std::ratio<1, 3>, yard::ratio>> {};
-  template<> struct upcasting_traits<upcast_from<foot>> : upcast_to<foot> {};
-
-  struct inch : unit<dimension_length, std::ratio_multiply<std::ratio<1, 12>, foot::ratio>> {};
-  template<> struct upcasting_traits<upcast_from<inch>> : upcast_to<inch> {};
-
-  struct mile : unit<dimension_length, std::ratio_multiply<std::ratio<1'760>, yard::ratio>> {};
-  template<> struct upcasting_traits<upcast_from<mile>> : upcast_to<mile> {};
-
-  // length
-  template<Unit U = meter, Number Rep = double>
-  using length = quantity<dimension_length, U, Rep>;
-
-  template<typename T>
-  concept bool Length = Quantity<T> && std::experimental::ranges::Same<typename T::dimension, dimension_length>;
-
   inline namespace literals {
 
     // mm
@@ -82,6 +67,23 @@ namespace units {
     constexpr auto operator""_km(unsigned long long l) { return length<kilometer, std::int64_t>(l); }
     constexpr auto operator""_km(long double l) { return length<kilometer, long double>(l); }
 
+  } // namespace literals
+
+  // US customary units
+  struct yard : unit<dimension_length, std::ratio<9'144, 10'000>> {};
+  template<> struct upcasting_traits<upcast_from<yard>> : upcast_to<yard> {};
+
+  struct foot : unit<dimension_length, std::ratio_multiply<std::ratio<1, 3>, yard::ratio>> {};
+  template<> struct upcasting_traits<upcast_from<foot>> : upcast_to<foot> {};
+
+  struct inch : unit<dimension_length, std::ratio_multiply<std::ratio<1, 12>, foot::ratio>> {};
+  template<> struct upcasting_traits<upcast_from<inch>> : upcast_to<inch> {};
+
+  struct mile : unit<dimension_length, std::ratio_multiply<std::ratio<1'760>, yard::ratio>> {};
+  template<> struct upcasting_traits<upcast_from<mile>> : upcast_to<mile> {};
+
+  inline namespace literals {
+
     // yd
     constexpr auto operator""_yd(unsigned long long l) { return length<yard, std::int64_t>(l); }
     constexpr auto operator""_yd(long double l) { return length<yard, long double>(l); }
diff --git a/src/include/units/luminous_intensity.h b/src/include/units/luminous_intensity.h
new file mode 100644
index 00000000..ac203384
--- /dev/null
+++ b/src/include/units/luminous_intensity.h
@@ -0,0 +1,50 @@
+// The MIT License (MIT)
+//
+// Copyright (c) 2018 Mateusz Pusz
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+#pragma once
+
+#include <units/base_dimensions.h>
+#include <units/quantity.h>
+
+namespace units {
+
+  struct dimension_luminous_intensity : make_dimension_t<exp<base_dim_luminous_intensity, 1>> {};
+  template<> struct upcasting_traits<upcast_from<dimension_luminous_intensity>> : upcast_to<dimension_luminous_intensity> {};
+
+  template<typename T>
+  concept bool LuminousIntensity = Quantity<T> && std::experimental::ranges::Same<typename T::dimension, dimension_luminous_intensity>;
+
+  template<Unit U = struct candela, Number Rep = double>
+  using luminous_intensity = quantity<dimension_luminous_intensity, U, Rep>;
+
+  struct candela : unit<dimension_luminous_intensity, std::ratio<1>> {};
+  template<> struct upcasting_traits<upcast_from<candela>> : upcast_to<candela> {};
+
+  inline namespace literals {
+
+    // cd
+    constexpr auto operator""_cd(unsigned long long l) { return luminous_intensity<candela, std::int64_t>(l); }
+    constexpr auto operator""_cd(long double l) { return luminous_intensity<candela, long double>(l); }
+
+  }  // namespace literals
+
+}  // namespace units
diff --git a/src/include/units/mass.h b/src/include/units/mass.h
new file mode 100644
index 00000000..d0672020
--- /dev/null
+++ b/src/include/units/mass.h
@@ -0,0 +1,53 @@
+// The MIT License (MIT)
+//
+// Copyright (c) 2018 Mateusz Pusz
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+#pragma once
+
+#include <units/base_dimensions.h>
+#include <units/quantity.h>
+
+namespace units {
+
+  struct dimension_mass : make_dimension_t<exp<base_dim_mass, 1>> {};
+  template<> struct upcasting_traits<upcast_from<dimension_mass>> : upcast_to<dimension_mass> {};
+
+  template<typename T>
+  concept bool Mass = Quantity<T> && std::experimental::ranges::Same<typename T::dimension, dimension_mass>;
+
+  template<Unit U = class kilogram, Number Rep = double>
+  using mass = quantity<dimension_mass, U, Rep>;
+
+  struct gram : unit<dimension_mass, std::milli> {};
+  template<> struct upcasting_traits<upcast_from<gram>> : upcast_to<gram> {};
+
+  struct kilogram : unit<dimension_mass, std::ratio<1>> {};
+  template<> struct upcasting_traits<upcast_from<kilogram>> : upcast_to<kilogram> {};
+
+  inline namespace literals {
+
+    // kg
+    constexpr auto operator""_kg(unsigned long long l) { return mass<kilogram, std::int64_t>(l); }
+    constexpr auto operator""_kg(long double l) { return mass<kilogram, long double>(l); }
+
+  }  // namespace literals
+
+}  // namespace units
diff --git a/src/include/units/quantity.h b/src/include/units/quantity.h
index e7e9ce51..d35b10bd 100644
--- a/src/include/units/quantity.h
+++ b/src/include/units/quantity.h
@@ -22,7 +22,7 @@
 
 #pragma once
 
-#include "unit.h"
+#include <units/unit.h>
 #include <limits>
 #include <gsl/gsl-lite.hpp>
 
diff --git a/src/include/units/substance.h b/src/include/units/substance.h
new file mode 100644
index 00000000..bc3e9b44
--- /dev/null
+++ b/src/include/units/substance.h
@@ -0,0 +1,50 @@
+// The MIT License (MIT)
+//
+// Copyright (c) 2018 Mateusz Pusz
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+#pragma once
+
+#include <units/base_dimensions.h>
+#include <units/quantity.h>
+
+namespace units {
+
+  struct dimension_substance : make_dimension_t<exp<base_dim_substance, 1>> {};
+  template<> struct upcasting_traits<upcast_from<dimension_substance>> : upcast_to<dimension_substance> {};
+
+  template<typename T>
+  concept bool Substance = Quantity<T> && std::experimental::ranges::Same<typename T::dimension, dimension_substance>;
+
+  template<Unit U = struct mole, Number Rep = double>
+  using substance = quantity<dimension_substance, U, Rep>;
+
+  struct mole : unit<dimension_substance, std::ratio<1>> {};
+  template<> struct upcasting_traits<upcast_from<mole>> : upcast_to<mole> {};
+
+  inline namespace literals {
+
+    // mol
+    constexpr auto operator""_mol(unsigned long long l) { return substance<mole, std::int64_t>(l); }
+    constexpr auto operator""_mol(long double l) { return substance<mole, long double>(l); }
+
+  }  // namespace literals
+
+}  // namespace units
diff --git a/src/include/units/temperature.h b/src/include/units/temperature.h
new file mode 100644
index 00000000..87414c1d
--- /dev/null
+++ b/src/include/units/temperature.h
@@ -0,0 +1,50 @@
+// The MIT License (MIT)
+//
+// Copyright (c) 2018 Mateusz Pusz
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+#pragma once
+
+#include <units/base_dimensions.h>
+#include <units/quantity.h>
+
+namespace units {
+
+  struct dimension_temperature : make_dimension_t<exp<base_dim_temperature, 1>> {};
+  template<> struct upcasting_traits<upcast_from<dimension_temperature>> : upcast_to<dimension_temperature> {};
+
+  template<typename T>
+  concept bool ThermodynamicTemperature = Quantity<T> && std::experimental::ranges::Same<typename T::dimension, dimension_temperature>;
+
+  template<Unit U = struct kelvin, Number Rep = double>
+  using temperature = quantity<dimension_temperature, U, Rep>;
+
+  struct kelvin : unit<dimension_temperature, std::ratio<1>> {};
+  template<> struct upcasting_traits<upcast_from<kelvin>> : upcast_to<kelvin> {};
+
+  inline namespace literals {
+
+    // K
+    constexpr auto operator""_K(unsigned long long l) { return temperature<kelvin, std::int64_t>(l); }
+    constexpr auto operator""_K(long double l) { return temperature<kelvin, long double>(l); }
+
+  }  // namespace literals
+
+}  // namespace units
diff --git a/src/include/units/si/time.h b/src/include/units/time.h
similarity index 96%
rename from src/include/units/si/time.h
rename to src/include/units/time.h
index 02e65bac..3602fa06 100644
--- a/src/include/units/si/time.h
+++ b/src/include/units/time.h
@@ -22,14 +22,20 @@
 
 #pragma once
 
-#include "base_dimensions.h"
-#include "../quantity.h"
+#include <units/base_dimensions.h>
+#include <units/quantity.h>
 
 namespace units {
 
   struct dimension_time : make_dimension_t<exp<base_dim_time, 1>> {};
   template<> struct upcasting_traits<upcast_from<dimension_time>> : upcast_to<dimension_time> {};
 
+  template<typename T>
+  concept bool Time = Quantity<T> && std::experimental::ranges::Same<typename T::dimension, dimension_time>;
+
+  template<Unit U = struct second, Number Rep = double>
+  using time = quantity<dimension_time, U, Rep>;
+
   struct nanosecond : unit<dimension_time, std::nano> {};
   template<> struct upcasting_traits<upcast_from<nanosecond>> : upcast_to<nanosecond> {};
 
@@ -48,13 +54,6 @@ namespace units {
   struct hour : unit<dimension_time, std::ratio<3600>> {};
   template<> struct upcasting_traits<upcast_from<hour>> : upcast_to<hour> {};
 
-  template<Unit U = second, Number Rep = double>
-  using time = quantity<dimension_time, U, Rep>;
-
-
-  template<typename T>
-  concept bool Time = Quantity<T> && std::experimental::ranges::Same<typename T::dimension, dimension_time>;
-
   inline namespace literals {
 
     // ns
diff --git a/src/include/units/unit.h b/src/include/units/unit.h
index 628a28ab..067a1364 100644
--- a/src/include/units/unit.h
+++ b/src/include/units/unit.h
@@ -22,8 +22,7 @@
 
 #pragma once
 
-#include "bits/tools.h"
-#include "dimension.h"
+#include <units/dimension.h>
 
 namespace units {
 
diff --git a/src/include/units/si/velocity.h b/src/include/units/velocity.h
similarity index 95%
rename from src/include/units/si/velocity.h
rename to src/include/units/velocity.h
index 379bfcc2..9cec999e 100644
--- a/src/include/units/si/velocity.h
+++ b/src/include/units/velocity.h
@@ -22,15 +22,21 @@
 
 #pragma once
 
-#include "base_dimensions.h"
-#include "length.h"
-#include "time.h"
+#include <units/base_dimensions.h>
+#include <units/length.h>
+#include <units/time.h>
 
 namespace units {
 
   struct dimension_velocity : make_dimension_t<exp<base_dim_length, 1>, exp<base_dim_time, -1>> {};
   template<> struct upcasting_traits<upcast_from<dimension_velocity>> : upcast_to<dimension_velocity> {};
 
+  template<typename T>
+  concept bool Velocity = Quantity<T> && std::experimental::ranges::Same<typename T::dimension, dimension_velocity>;
+
+  template<Unit U = struct meter_per_second, Number Rep = double>
+  using velocity = quantity<dimension_velocity, U, Rep>;
+
   struct meter_per_second : unit<dimension_velocity, std::ratio<1>> {};
   template<> struct upcasting_traits<upcast_from<meter_per_second>> : upcast_to<meter_per_second> {};
 
@@ -40,12 +46,6 @@ namespace units {
   struct mile_per_hour : unit<dimension_velocity, std::ratio_divide<mile::ratio, hour::ratio>> {};
   template<> struct upcasting_traits<upcast_from<mile_per_hour>> : upcast_to<mile_per_hour> {};
 
-  template<Unit U = meter_per_second, Number Rep = double>
-  using velocity = quantity<dimension_velocity, U, Rep>;
-
-  template<typename T>
-  concept bool Velocity = Quantity<T> && std::experimental::ranges::Same<typename T::dimension, dimension_velocity>;
-
   inline namespace literals {
 
     // mps
diff --git a/test/test_quantity.cpp b/test/test_quantity.cpp
index 9bb00ed9..f240d814 100644
--- a/test/test_quantity.cpp
+++ b/test/test_quantity.cpp
@@ -20,8 +20,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
-#include "units/si/velocity.h"
-#include "units/si/frequency.h"
+#include "units/velocity.h"
+#include "units/frequency.h"
 #include <utility>
 #include <chrono>
 
diff --git a/test/test_units.cpp b/test/test_units.cpp
index 9900a73f..aa8cf785 100644
--- a/test/test_units.cpp
+++ b/test/test_units.cpp
@@ -20,10 +20,18 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
-#include "units/si/frequency.h"
-#include "units/si/length.h"
-#include "units/si/time.h"
-#include "units/si/velocity.h"
+#include <units/time.h>
+#include <units/length.h>
+#include <units/mass.h>
+#include <units/current.h>
+#include <units/current.h>
+#include <units/temperature.h>
+#include <units/substance.h>
+#include <units/luminous_intensity.h>
+
+#include <units/frequency.h>
+#include <units/velocity.h>
+
 #include <utility>
 
 namespace {