diff --git a/example/avg_speed.cpp b/example/avg_speed.cpp
index 2ccc1ef1..1a578fab 100644
--- a/example/avg_speed.cpp
+++ b/example/avg_speed.cpp
@@ -20,11 +20,11 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
-#include <units/cgs/cgs.h>
-#include <units/isq/space_and_time.h>
 #include <units/quantity_io.h>
-#include <units/si/unit_symbols.h>
-#include <units/international/international.h>
+#include <units/systems/cgs/cgs.h>
+#include <units/systems/international/international.h>
+#include <units/systems/isq/space_and_time.h>
+#include <units/systems/si/unit_symbols.h>
 #include <exception>
 #include <iostream>
 
diff --git a/example/box_example.cpp b/example/box_example.cpp
index 9ff5a40d..9c710a4b 100644
--- a/example/box_example.cpp
+++ b/example/box_example.cpp
@@ -21,11 +21,11 @@
 // SOFTWARE.
 
 #include <units/format.h>
-#include <units/isq/mechanics.h>
-#include <units/isq/space_and_time.h>
-#include <units/si/constants.h>
-#include <units/si/unit_symbols.h>
-#include <units/si/units.h>
+#include <units/systems/isq/mechanics.h>
+#include <units/systems/isq/space_and_time.h>
+#include <units/systems/si/constants.h>
+#include <units/systems/si/unit_symbols.h>
+#include <units/systems/si/units.h>
 #include <cassert>
 #include <iostream>
 #include <utility>
diff --git a/example/capacitor_time_curve.cpp b/example/capacitor_time_curve.cpp
index f453eb66..b538a114 100644
--- a/example/capacitor_time_curve.cpp
+++ b/example/capacitor_time_curve.cpp
@@ -20,10 +20,10 @@
     physical_quantities
 */
 
-#include <units/isq/electromagnetism.h>
 #include <units/math.h>  // IWYU pragma: keep
 #include <units/quantity_io.h>
-#include <units/si/si.h>
+#include <units/systems/isq/electromagnetism.h>
+#include <units/systems/si/si.h>
 #include <iostream>
 
 int main()
diff --git a/example/clcpp_response.cpp b/example/clcpp_response.cpp
index a572bd90..5ada0522 100644
--- a/example/clcpp_response.cpp
+++ b/example/clcpp_response.cpp
@@ -16,14 +16,14 @@
 */
 
 #include <units/format.h>
-#include <units/iau/iau.h>
-#include <units/imperial/imperial.h>
-#include <units/international/international.h>
-#include <units/isq/space_and_time.h>
 #include <units/quantity_io.h>
-#include <units/si/si.h>
-#include <units/typographic/typographic.h>
-#include <units/usc/usc.h>
+#include <units/systems/iau/iau.h>
+#include <units/systems/imperial/imperial.h>
+#include <units/systems/international/international.h>
+#include <units/systems/isq/space_and_time.h>
+#include <units/systems/si/si.h>
+#include <units/systems/typographic/typographic.h>
+#include <units/systems/usc/usc.h>
 #include <iostream>
 
 namespace {
diff --git a/example/conversion_factor.cpp b/example/conversion_factor.cpp
index 263f896a..a07e3371 100644
--- a/example/conversion_factor.cpp
+++ b/example/conversion_factor.cpp
@@ -16,8 +16,8 @@
 */
 
 #include <units/format.h>
-#include <units/isq/space_and_time.h>
-#include <units/si/units.h>
+#include <units/systems/isq/space_and_time.h>
+#include <units/systems/si/units.h>
 #include <iostream>
 #include <type_traits>
 
diff --git a/example/foot_pound_second.cpp b/example/foot_pound_second.cpp
index 0d0cbb02..a404992d 100644
--- a/example/foot_pound_second.cpp
+++ b/example/foot_pound_second.cpp
@@ -21,10 +21,10 @@
 // SOFTWARE.
 
 #include <units/format.h>
-#include <units/imperial/imperial.h>
-#include <units/international/international.h>
-#include <units/isq/space_and_time.h>
-#include <units/si/unit_symbols.h>
+#include <units/systems/imperial/imperial.h>
+#include <units/systems/international/international.h>
+#include <units/systems/isq/space_and_time.h>
+#include <units/systems/si/unit_symbols.h>
 #include <iostream>
 #include <string_view>
 
diff --git a/example/glide_computer/include/geographic.h b/example/glide_computer/include/geographic.h
index 094c2520..e0a597d4 100644
--- a/example/glide_computer/include/geographic.h
+++ b/example/glide_computer/include/geographic.h
@@ -24,9 +24,9 @@
 
 #include "ranged_representation.h"
 #include <units/bits/fmt_hacks.h>
-#include <units/isq/space_and_time.h>
 #include <units/quantity.h>
-#include <units/si/units.h>
+#include <units/systems/isq/space_and_time.h>
+#include <units/systems/si/units.h>
 #include <compare>
 #include <limits>
 #include <numbers>
diff --git a/example/glide_computer/include/glide_computer.h b/example/glide_computer/include/glide_computer.h
index 0f9d8358..7c34daa0 100644
--- a/example/glide_computer/include/glide_computer.h
+++ b/example/glide_computer/include/glide_computer.h
@@ -25,9 +25,9 @@
 #include "geographic.h"
 #include <units/chrono.h>
 #include <units/format.h>
-#include <units/isq/space_and_time.h>
 #include <units/math.h>  // IWYU pragma: keep
 #include <units/quantity_point.h>
+#include <units/systems/isq/space_and_time.h>
 #include <algorithm>
 #include <array>
 #include <initializer_list>
diff --git a/example/glide_computer_example.cpp b/example/glide_computer_example.cpp
index 10f7ae8b..3b0cf7d8 100644
--- a/example/glide_computer_example.cpp
+++ b/example/glide_computer_example.cpp
@@ -23,9 +23,9 @@
 #include "glide_computer.h"
 #include <units/bits/fmt_hacks.h>
 #include <units/chrono.h>
-#include <units/international/international.h>
+#include <units/systems/international/international.h>
 #include <units/math.h>
-#include <units/si/unit_symbols.h>
+#include <units/systems/si/unit_symbols.h>
 #include <array>
 #include <exception>
 #include <iostream>
diff --git a/example/hello_units.cpp b/example/hello_units.cpp
index 743c415d..0e0d6557 100644
--- a/example/hello_units.cpp
+++ b/example/hello_units.cpp
@@ -21,10 +21,10 @@
 // SOFTWARE.
 
 #include <units/format.h>
-#include <units/international/international.h>
-#include <units/isq/space_and_time.h>
+#include <units/systems/international/international.h>
+#include <units/systems/isq/space_and_time.h>
 #include <units/quantity_io.h>
-#include <units/si/unit_symbols.h>
+#include <units/systems/si/unit_symbols.h>
 #include <iostream>
 
 using namespace units;
diff --git a/example/kalman_filter/kalman.h b/example/kalman_filter/kalman.h
index e9341d43..121c5b4e 100644
--- a/example/kalman_filter/kalman.h
+++ b/example/kalman_filter/kalman.h
@@ -24,10 +24,10 @@
 
 #include <units/bits/fmt_hacks.h>
 #include <units/format.h>
-#include <units/isq/space_and_time.h>
 #include <units/math.h>
 #include <units/quantity.h>
 #include <units/quantity_point.h>
+#include <units/systems/isq/space_and_time.h>
 #include <tuple>
 
 namespace kalman {
diff --git a/example/kalman_filter/kalman_filter-example_1.cpp b/example/kalman_filter/kalman_filter-example_1.cpp
index 47d8f6b7..f9b4bfa7 100644
--- a/example/kalman_filter/kalman_filter-example_1.cpp
+++ b/example/kalman_filter/kalman_filter-example_1.cpp
@@ -22,8 +22,8 @@
 
 #include "kalman.h"
 #include <units/format.h>
-#include <units/isq/space_and_time.h>
-#include <units/si/unit_symbols.h>
+#include <units/systems/isq/space_and_time.h>
+#include <units/systems/si/unit_symbols.h>
 #include <array>
 #include <iostream>
 
diff --git a/example/kalman_filter/kalman_filter-example_2.cpp b/example/kalman_filter/kalman_filter-example_2.cpp
index b188c250..b480c08b 100644
--- a/example/kalman_filter/kalman_filter-example_2.cpp
+++ b/example/kalman_filter/kalman_filter-example_2.cpp
@@ -22,8 +22,8 @@
 
 #include "kalman.h"
 #include <units/format.h>
-#include <units/isq/space_and_time.h>
-#include <units/si/unit_symbols.h>
+#include <units/systems/isq/space_and_time.h>
+#include <units/systems/si/unit_symbols.h>
 #include <array>
 #include <iostream>
 
diff --git a/example/kalman_filter/kalman_filter-example_3.cpp b/example/kalman_filter/kalman_filter-example_3.cpp
index 193526f7..8872e014 100644
--- a/example/kalman_filter/kalman_filter-example_3.cpp
+++ b/example/kalman_filter/kalman_filter-example_3.cpp
@@ -22,8 +22,8 @@
 
 #include "kalman.h"
 #include <units/format.h>
-#include <units/isq/space_and_time.h>
-#include <units/si/unit_symbols.h>
+#include <units/systems/isq/space_and_time.h>
+#include <units/systems/si/unit_symbols.h>
 #include <array>
 #include <iostream>
 
diff --git a/example/kalman_filter/kalman_filter-example_4.cpp b/example/kalman_filter/kalman_filter-example_4.cpp
index 34c27e7a..5b3dc698 100644
--- a/example/kalman_filter/kalman_filter-example_4.cpp
+++ b/example/kalman_filter/kalman_filter-example_4.cpp
@@ -22,8 +22,8 @@
 
 #include "kalman.h"
 #include <units/format.h>
-#include <units/isq/space_and_time.h>
-#include <units/si/unit_symbols.h>
+#include <units/systems/isq/space_and_time.h>
+#include <units/systems/si/unit_symbols.h>
 #include <array>
 #include <iostream>
 
diff --git a/example/kalman_filter/kalman_filter-example_5.cpp b/example/kalman_filter/kalman_filter-example_5.cpp
index 4efcc0ce..f420aa1b 100644
--- a/example/kalman_filter/kalman_filter-example_5.cpp
+++ b/example/kalman_filter/kalman_filter-example_5.cpp
@@ -22,9 +22,9 @@
 
 #include "kalman.h"
 #include <units/format.h>
-#include <units/isq/space_and_time.h>
 #include <units/math.h>
-#include <units/si/unit_symbols.h>
+#include <units/systems/isq/space_and_time.h>
+#include <units/systems/si/unit_symbols.h>
 #include <array>
 #include <iostream>
 
diff --git a/example/kalman_filter/kalman_filter-example_6.cpp b/example/kalman_filter/kalman_filter-example_6.cpp
index 507a1b16..e622d84b 100644
--- a/example/kalman_filter/kalman_filter-example_6.cpp
+++ b/example/kalman_filter/kalman_filter-example_6.cpp
@@ -22,10 +22,10 @@
 
 #include "kalman.h"
 #include <units/format.h>
-#include <units/isq/thermodynamics.h>
 #include <units/math.h>
 #include <units/quantity_point.h>
-#include <units/si/unit_symbols.h>
+#include <units/systems/isq/thermodynamics.h>
+#include <units/systems/si/unit_symbols.h>
 #include <array>
 #include <iostream>
 
diff --git a/example/kalman_filter/kalman_filter-example_7.cpp b/example/kalman_filter/kalman_filter-example_7.cpp
index d0f53585..816a8561 100644
--- a/example/kalman_filter/kalman_filter-example_7.cpp
+++ b/example/kalman_filter/kalman_filter-example_7.cpp
@@ -22,10 +22,10 @@
 
 #include "kalman.h"
 #include <units/format.h>
-#include <units/isq/thermodynamics.h>
 #include <units/math.h>
 #include <units/quantity_point.h>
-#include <units/si/unit_symbols.h>
+#include <units/systems/isq/thermodynamics.h>
+#include <units/systems/si/unit_symbols.h>
 #include <array>
 #include <iostream>
 
diff --git a/example/kalman_filter/kalman_filter-example_8.cpp b/example/kalman_filter/kalman_filter-example_8.cpp
index 5b7df927..29191450 100644
--- a/example/kalman_filter/kalman_filter-example_8.cpp
+++ b/example/kalman_filter/kalman_filter-example_8.cpp
@@ -22,10 +22,10 @@
 
 #include "kalman.h"
 #include <units/format.h>
-#include <units/isq/thermodynamics.h>
 #include <units/math.h>
 #include <units/quantity_point.h>
-#include <units/si/unit_symbols.h>
+#include <units/systems/isq/thermodynamics.h>
+#include <units/systems/si/unit_symbols.h>
 #include <array>
 #include <iostream>
 
@@ -63,7 +63,8 @@ int main()
                                    quantity_point{54.523 * deg_C}, quantity_point{54.99 * deg_C}};
   const auto measurement_uncertainty = pow<2>(0.1 * deg_C);
 
-  auto update = [=]<QuantityPoint QP>(const estimation<QP>& previous, const QP& meassurement, quantity_of<dimensionless> auto gain) {
+  auto update = [=]<QuantityPoint QP>(const estimation<QP>& previous, const QP& meassurement,
+                                      quantity_of<dimensionless> auto gain) {
     return estimation{state_update(previous.state, meassurement, gain), covariance_update(previous.uncertainty, gain)};
   };
 
diff --git a/example/measurement.cpp b/example/measurement.cpp
index 281e40c6..c9fcef33 100644
--- a/example/measurement.cpp
+++ b/example/measurement.cpp
@@ -20,10 +20,10 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
-#include <units/isq/space_and_time.h>
+#include <units/systems/isq/space_and_time.h>
 #include <units/quantity_io.h>
-#include <units/si/unit_symbols.h>
-#include <units/si/units.h>
+#include <units/systems/si/unit_symbols.h>
+#include <units/systems/si/units.h>
 #include <cmath>
 #include <exception>
 #include <iostream>
diff --git a/example/si_constants.cpp b/example/si_constants.cpp
index 45647710..cc5a782b 100644
--- a/example/si_constants.cpp
+++ b/example/si_constants.cpp
@@ -21,8 +21,8 @@
 // SOFTWARE.
 
 #include <units/format.h>
-#include <units/si/constants.h>
-#include <units/si/unit_symbols.h>
+#include <units/systems/si/constants.h>
+#include <units/systems/si/unit_symbols.h>
 #include <iostream>
 
 template<class T>
diff --git a/example/strong_angular_quantities.cpp b/example/strong_angular_quantities.cpp
index c80c9cfc..e08eab02 100644
--- a/example/strong_angular_quantities.cpp
+++ b/example/strong_angular_quantities.cpp
@@ -20,10 +20,10 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
-#include <units/isq_angle/isq_angle.h>
 #include <units/math.h>
 #include <units/quantity_io.h>
-#include <units/si/unit_symbols.h>
+#include <units/systems/isq_angle/isq_angle.h>
+#include <units/systems/si/unit_symbols.h>
 #include <iostream>
 
 template<class T>
diff --git a/example/total_energy.cpp b/example/total_energy.cpp
index 2d4741c0..77034388 100644
--- a/example/total_energy.cpp
+++ b/example/total_energy.cpp
@@ -20,12 +20,12 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
-#include <units/isq/mechanics.h>
 #include <units/math.h>
-#include <units/natural/natural.h>
 #include <units/quantity_io.h>
-#include <units/si/constants.h>
-#include <units/si/unit_symbols.h>
+#include <units/systems/isq/mechanics.h>
+#include <units/systems/natural/natural.h>
+#include <units/systems/si/constants.h>
+#include <units/systems/si/unit_symbols.h>
 #include <exception>
 #include <iostream>
 
diff --git a/src/systems/angular/CMakeLists.txt b/src/systems/angular/CMakeLists.txt
index 83c32057..0cb3f2cc 100644
--- a/src/systems/angular/CMakeLists.txt
+++ b/src/systems/angular/CMakeLists.txt
@@ -22,4 +22,4 @@
 
 cmake_minimum_required(VERSION 3.19)
 
-add_units_module(angular DEPENDENCIES mp-units::isq HEADERS include/units/angular/angular.h)
+add_units_module(angular DEPENDENCIES mp-units::isq HEADERS include/units/systems/angular/angular.h)
diff --git a/src/systems/angular/include/units/angular/angular.h b/src/systems/angular/include/units/systems/angular/angular.h
similarity index 100%
rename from src/systems/angular/include/units/angular/angular.h
rename to src/systems/angular/include/units/systems/angular/angular.h
diff --git a/src/systems/cgs/CMakeLists.txt b/src/systems/cgs/CMakeLists.txt
index 5c5f8889..31a6964d 100644
--- a/src/systems/cgs/CMakeLists.txt
+++ b/src/systems/cgs/CMakeLists.txt
@@ -22,4 +22,4 @@
 
 cmake_minimum_required(VERSION 3.19)
 
-add_units_module(cgs DEPENDENCIES mp-units::si HEADERS include/units/cgs/cgs.h)
+add_units_module(cgs DEPENDENCIES mp-units::si HEADERS include/units/systems/cgs/cgs.h)
diff --git a/src/systems/cgs/include/units/cgs/cgs.h b/src/systems/cgs/include/units/systems/cgs/cgs.h
similarity index 98%
rename from src/systems/cgs/include/units/cgs/cgs.h
rename to src/systems/cgs/include/units/systems/cgs/cgs.h
index ebb80ae6..df81e3fa 100644
--- a/src/systems/cgs/include/units/cgs/cgs.h
+++ b/src/systems/cgs/include/units/systems/cgs/cgs.h
@@ -22,7 +22,7 @@
 
 #pragma once
 
-#include <units/si/units.h>
+#include <units/systems/si/units.h>
 #include <units/unit.h>
 
 namespace units::cgs {
diff --git a/src/systems/hep/CMakeLists.txt b/src/systems/hep/CMakeLists.txt
index d778901b..14ab6df3 100644
--- a/src/systems/hep/CMakeLists.txt
+++ b/src/systems/hep/CMakeLists.txt
@@ -22,4 +22,4 @@
 
 cmake_minimum_required(VERSION 3.19)
 
-add_units_module(hep DEPENDENCIES mp-units::si HEADERS include/units/hep/hep.h)
+add_units_module(hep DEPENDENCIES mp-units::si HEADERS include/units/systems/hep/hep.h)
diff --git a/src/systems/hep/include/units/hep/hep.h b/src/systems/hep/include/units/systems/hep/hep.h
similarity index 99%
rename from src/systems/hep/include/units/hep/hep.h
rename to src/systems/hep/include/units/systems/hep/hep.h
index df277f4e..c57158ac 100644
--- a/src/systems/hep/include/units/hep/hep.h
+++ b/src/systems/hep/include/units/systems/hep/hep.h
@@ -22,7 +22,7 @@
 
 #pragma once
 
-#include <units/si/si.h>
+#include <units/systems/si/si.h>
 #include <units/unit.h>
 
 namespace units::hep {
diff --git a/src/systems/iau/CMakeLists.txt b/src/systems/iau/CMakeLists.txt
index 3ac37221..edd95a2b 100644
--- a/src/systems/iau/CMakeLists.txt
+++ b/src/systems/iau/CMakeLists.txt
@@ -23,5 +23,5 @@
 cmake_minimum_required(VERSION 3.19)
 
 add_units_module(
-    iau DEPENDENCIES mp-units::si HEADERS include/units/iau/iau.h
+    iau DEPENDENCIES mp-units::si HEADERS include/units/systems/iau/iau.h
 )
diff --git a/src/systems/iau/include/units/iau/iau.h b/src/systems/iau/include/units/systems/iau/iau.h
similarity index 99%
rename from src/systems/iau/include/units/iau/iau.h
rename to src/systems/iau/include/units/systems/iau/iau.h
index 067173e1..87b1f447 100644
--- a/src/systems/iau/include/units/iau/iau.h
+++ b/src/systems/iau/include/units/systems/iau/iau.h
@@ -22,7 +22,7 @@
 
 #pragma once
 
-#include <units/si/si.h>
+#include <units/systems/si/si.h>
 #include <units/unit.h>
 
 namespace units::iau {
diff --git a/src/systems/iec80000/CMakeLists.txt b/src/systems/iec80000/CMakeLists.txt
index 592b9030..55afba34 100644
--- a/src/systems/iec80000/CMakeLists.txt
+++ b/src/systems/iec80000/CMakeLists.txt
@@ -24,6 +24,6 @@ cmake_minimum_required(VERSION 3.19)
 
 add_units_module(
     iec80000 DEPENDENCIES mp-units::isq mp-units::si
-    HEADERS include/units/iec80000/binary_prefixes.h include/units/iec80000/quantities.h
-            include/units/iec80000/iec80000.h include/units/iec80000/unit_symbols.h include/units/iec80000/units.h
+    HEADERS include/units/systems/iec80000/binary_prefixes.h include/units/systems/iec80000/quantities.h
+    include/units/systems/iec80000/iec80000.h include/units/systems/iec80000/unit_symbols.h include/units/systems/iec80000/units.h
 )
diff --git a/src/systems/iec80000/include/units/iec80000/binary_prefixes.h b/src/systems/iec80000/include/units/systems/iec80000/binary_prefixes.h
similarity index 100%
rename from src/systems/iec80000/include/units/iec80000/binary_prefixes.h
rename to src/systems/iec80000/include/units/systems/iec80000/binary_prefixes.h
diff --git a/src/systems/iec80000/include/units/iec80000/iec80000.h b/src/systems/iec80000/include/units/systems/iec80000/iec80000.h
similarity index 86%
rename from src/systems/iec80000/include/units/iec80000/iec80000.h
rename to src/systems/iec80000/include/units/systems/iec80000/iec80000.h
index eed775bd..702d71c4 100644
--- a/src/systems/iec80000/include/units/iec80000/iec80000.h
+++ b/src/systems/iec80000/include/units/systems/iec80000/iec80000.h
@@ -23,8 +23,8 @@
 #pragma once
 
 // IWYU pragma: begin_exports
-#include <units/iec80000/binary_prefixes.h>
-#include <units/iec80000/quantities.h>
-#include <units/iec80000/unit_symbols.h>
-#include <units/iec80000/units.h>
+#include <units/systems/iec80000/binary_prefixes.h>
+#include <units/systems/iec80000/quantities.h>
+#include <units/systems/iec80000/unit_symbols.h>
+#include <units/systems/iec80000/units.h>
 // IWYU pragma: end_exports
diff --git a/src/systems/iec80000/include/units/iec80000/quantities.h b/src/systems/iec80000/include/units/systems/iec80000/quantities.h
similarity index 97%
rename from src/systems/iec80000/include/units/iec80000/quantities.h
rename to src/systems/iec80000/include/units/systems/iec80000/quantities.h
index ee479547..7bd8fb2c 100644
--- a/src/systems/iec80000/include/units/iec80000/quantities.h
+++ b/src/systems/iec80000/include/units/systems/iec80000/quantities.h
@@ -23,9 +23,9 @@
 #pragma once
 
 #include <units/dimension.h>
-#include <units/isq/mechanics.h>
-#include <units/isq/space_and_time.h>
 #include <units/quantity_spec.h>
+#include <units/systems/isq/mechanics.h>
+#include <units/systems/isq/space_and_time.h>
 
 namespace units::iec80000 {
 
diff --git a/src/systems/iec80000/include/units/iec80000/unit_symbols.h b/src/systems/iec80000/include/units/systems/iec80000/unit_symbols.h
similarity index 96%
rename from src/systems/iec80000/include/units/iec80000/unit_symbols.h
rename to src/systems/iec80000/include/units/systems/iec80000/unit_symbols.h
index 25d350df..d96f19b8 100644
--- a/src/systems/iec80000/include/units/iec80000/unit_symbols.h
+++ b/src/systems/iec80000/include/units/systems/iec80000/unit_symbols.h
@@ -22,9 +22,9 @@
 
 #pragma once
 
-#include <units/iec80000/binary_prefixes.h>
-#include <units/iec80000/units.h>
-#include <units/si/prefixes.h>
+#include <units/systems/iec80000/binary_prefixes.h>
+#include <units/systems/iec80000/units.h>
+#include <units/systems/si/prefixes.h>
 
 namespace units::iec80000::unit_symbols {
 
diff --git a/src/systems/iec80000/include/units/iec80000/units.h b/src/systems/iec80000/include/units/systems/iec80000/units.h
similarity index 95%
rename from src/systems/iec80000/include/units/iec80000/units.h
rename to src/systems/iec80000/include/units/systems/iec80000/units.h
index b3e6ffc3..3aa40b12 100644
--- a/src/systems/iec80000/include/units/iec80000/units.h
+++ b/src/systems/iec80000/include/units/systems/iec80000/units.h
@@ -22,8 +22,8 @@
 
 #pragma once
 
-#include <units/iec80000/quantities.h>
-#include <units/si/units.h>
+#include <units/systems/iec80000/quantities.h>
+#include <units/systems/si/units.h>
 #include <units/unit.h>
 
 namespace units::iec80000 {
diff --git a/src/systems/imperial/CMakeLists.txt b/src/systems/imperial/CMakeLists.txt
index 66a926f2..2915d6b5 100644
--- a/src/systems/imperial/CMakeLists.txt
+++ b/src/systems/imperial/CMakeLists.txt
@@ -22,4 +22,4 @@
 
 cmake_minimum_required(VERSION 3.19)
 
-add_units_module(imperial DEPENDENCIES mp-units::si mp-units::international HEADERS include/units/imperial/imperial.h)
+add_units_module(imperial DEPENDENCIES mp-units::si mp-units::international HEADERS include/units/systems/imperial/imperial.h)
diff --git a/src/systems/imperial/include/units/imperial/imperial.h b/src/systems/imperial/include/units/systems/imperial/imperial.h
similarity index 98%
rename from src/systems/imperial/include/units/imperial/imperial.h
rename to src/systems/imperial/include/units/systems/imperial/imperial.h
index 65374ed1..9c1c41cc 100644
--- a/src/systems/imperial/include/units/imperial/imperial.h
+++ b/src/systems/imperial/include/units/systems/imperial/imperial.h
@@ -22,7 +22,7 @@
 
 #pragma once
 
-#include <units/international/international.h>
+#include <units/systems/international/international.h>
 #include <units/unit.h>
 
 namespace units::imperial {
diff --git a/src/systems/international/CMakeLists.txt b/src/systems/international/CMakeLists.txt
index 27bd41e5..b4506398 100644
--- a/src/systems/international/CMakeLists.txt
+++ b/src/systems/international/CMakeLists.txt
@@ -22,4 +22,4 @@
 
 cmake_minimum_required(VERSION 3.19)
 
-add_units_module(international DEPENDENCIES mp-units::si HEADERS include/units/international/international.h)
+add_units_module(international DEPENDENCIES mp-units::si HEADERS include/units/systems/international/international.h)
diff --git a/src/systems/international/include/units/international/international.h b/src/systems/international/include/units/systems/international/international.h
similarity index 99%
rename from src/systems/international/include/units/international/international.h
rename to src/systems/international/include/units/systems/international/international.h
index b37af7d9..4b61f77f 100644
--- a/src/systems/international/include/units/international/international.h
+++ b/src/systems/international/include/units/systems/international/international.h
@@ -22,7 +22,7 @@
 
 #pragma once
 
-#include <units/si/si.h>
+#include <units/systems/si/si.h>
 #include <units/unit.h>
 
 namespace units::international {
diff --git a/src/systems/isq/CMakeLists.txt b/src/systems/isq/CMakeLists.txt
index 84fa3542..68be7f0c 100644
--- a/src/systems/isq/CMakeLists.txt
+++ b/src/systems/isq/CMakeLists.txt
@@ -24,6 +24,6 @@ cmake_minimum_required(VERSION 3.19)
 
 add_units_module(
     isq DEPENDENCIES mp-units::core
-    HEADERS include/units/isq/base_quantities.h include/units/isq/electromagnetism.h include/units/isq/isq.h
-            include/units/isq/mechanics.h include/units/isq/space_and_time.h include/units/isq/thermodynamics.h
+    HEADERS include/units/systems/isq/base_quantities.h include/units/systems/isq/electromagnetism.h include/units/systems/isq/isq.h
+            include/units/systems/isq/mechanics.h include/units/systems/isq/space_and_time.h include/units/systems/isq/thermodynamics.h
 )
diff --git a/src/systems/isq/include/units/isq/base_quantities.h b/src/systems/isq/include/units/systems/isq/base_quantities.h
similarity index 100%
rename from src/systems/isq/include/units/isq/base_quantities.h
rename to src/systems/isq/include/units/systems/isq/base_quantities.h
diff --git a/src/systems/isq/include/units/isq/electromagnetism.h b/src/systems/isq/include/units/systems/isq/electromagnetism.h
similarity index 98%
rename from src/systems/isq/include/units/isq/electromagnetism.h
rename to src/systems/isq/include/units/systems/isq/electromagnetism.h
index fbe2bd69..67804907 100644
--- a/src/systems/isq/include/units/isq/electromagnetism.h
+++ b/src/systems/isq/include/units/systems/isq/electromagnetism.h
@@ -22,9 +22,9 @@
 
 #pragma once
 
-#include <units/isq/base_quantities.h>
-#include <units/isq/mechanics.h>
-#include <units/isq/space_and_time.h>
+#include <units/systems/isq/base_quantities.h>
+#include <units/systems/isq/mechanics.h>
+#include <units/systems/isq/space_and_time.h>
 #include <units/quantity_spec.h>
 
 namespace units::isq {
diff --git a/src/systems/isq/include/units/isq/isq.h b/src/systems/isq/include/units/systems/isq/isq.h
similarity index 84%
rename from src/systems/isq/include/units/isq/isq.h
rename to src/systems/isq/include/units/systems/isq/isq.h
index 66986675..5d824e51 100644
--- a/src/systems/isq/include/units/isq/isq.h
+++ b/src/systems/isq/include/units/systems/isq/isq.h
@@ -23,9 +23,9 @@
 #pragma once
 
 // IWYU pragma: begin_exports
-#include <units/isq/base_quantities.h>
-#include <units/isq/electromagnetism.h>
-#include <units/isq/mechanics.h>
-#include <units/isq/space_and_time.h>
-#include <units/isq/thermodynamics.h>
+#include <units/systems/isq/base_quantities.h>
+#include <units/systems/isq/electromagnetism.h>
+#include <units/systems/isq/mechanics.h>
+#include <units/systems/isq/space_and_time.h>
+#include <units/systems/isq/thermodynamics.h>
 // IWYU pragma: end_exports
diff --git a/src/systems/isq/include/units/isq/mechanics.h b/src/systems/isq/include/units/systems/isq/mechanics.h
similarity index 98%
rename from src/systems/isq/include/units/isq/mechanics.h
rename to src/systems/isq/include/units/systems/isq/mechanics.h
index dad811d0..1bb4e6d0 100644
--- a/src/systems/isq/include/units/isq/mechanics.h
+++ b/src/systems/isq/include/units/systems/isq/mechanics.h
@@ -22,8 +22,8 @@
 
 #pragma once
 
-#include <units/isq/base_quantities.h>
-#include <units/isq/space_and_time.h>
+#include <units/systems/isq/base_quantities.h>
+#include <units/systems/isq/space_and_time.h>
 #include <units/quantity_spec.h>
 
 namespace units::isq {
diff --git a/src/systems/isq/include/units/isq/space_and_time.h b/src/systems/isq/include/units/systems/isq/space_and_time.h
similarity index 98%
rename from src/systems/isq/include/units/isq/space_and_time.h
rename to src/systems/isq/include/units/systems/isq/space_and_time.h
index f45c550f..c6c99cfd 100644
--- a/src/systems/isq/include/units/isq/space_and_time.h
+++ b/src/systems/isq/include/units/systems/isq/space_and_time.h
@@ -22,7 +22,7 @@
 
 #pragma once
 
-#include <units/isq/base_quantities.h>
+#include <units/systems/isq/base_quantities.h>
 #include <units/quantity_spec.h>
 
 namespace units::isq {
diff --git a/src/systems/isq/include/units/isq/thermodynamics.h b/src/systems/isq/include/units/systems/isq/thermodynamics.h
similarity index 97%
rename from src/systems/isq/include/units/isq/thermodynamics.h
rename to src/systems/isq/include/units/systems/isq/thermodynamics.h
index e7bee2ae..ecf84851 100644
--- a/src/systems/isq/include/units/isq/thermodynamics.h
+++ b/src/systems/isq/include/units/systems/isq/thermodynamics.h
@@ -22,10 +22,10 @@
 
 #pragma once
 
-#include <units/isq/base_quantities.h>
-#include <units/isq/mechanics.h>
-#include <units/isq/space_and_time.h>
 #include <units/quantity_spec.h>
+#include <units/systems/isq/base_quantities.h>
+#include <units/systems/isq/mechanics.h>
+#include <units/systems/isq/space_and_time.h>
 
 namespace units::isq {
 
diff --git a/src/systems/isq_angle/CMakeLists.txt b/src/systems/isq_angle/CMakeLists.txt
index 72fdaec1..8f4ae36e 100644
--- a/src/systems/isq_angle/CMakeLists.txt
+++ b/src/systems/isq_angle/CMakeLists.txt
@@ -22,4 +22,4 @@
 
 cmake_minimum_required(VERSION 3.19)
 
-add_units_module(isq_angle DEPENDENCIES mp-units::angular mp-units::isq HEADERS include/units/isq_angle/isq_angle.h)
+add_units_module(isq_angle DEPENDENCIES mp-units::angular mp-units::isq HEADERS include/units/systems/isq_angle/isq_angle.h)
diff --git a/src/systems/isq_angle/include/units/isq_angle/isq_angle.h b/src/systems/isq_angle/include/units/systems/isq_angle/isq_angle.h
similarity index 97%
rename from src/systems/isq_angle/include/units/isq_angle/isq_angle.h
rename to src/systems/isq_angle/include/units/systems/isq_angle/isq_angle.h
index 42340fde..14ce0392 100644
--- a/src/systems/isq_angle/include/units/isq_angle/isq_angle.h
+++ b/src/systems/isq_angle/include/units/systems/isq_angle/isq_angle.h
@@ -22,9 +22,9 @@
 
 #pragma once
 
-#include <units/angular/angular.h>
-#include <units/isq/isq.h>
 #include <units/quantity_spec.h>
+#include <units/systems/angular/angular.h>
+#include <units/systems/isq/isq.h>
 
 namespace units::isq_angle {
 
diff --git a/src/systems/natural/CMakeLists.txt b/src/systems/natural/CMakeLists.txt
index 0684f221..884319cf 100644
--- a/src/systems/natural/CMakeLists.txt
+++ b/src/systems/natural/CMakeLists.txt
@@ -22,4 +22,4 @@
 
 cmake_minimum_required(VERSION 3.19)
 
-add_units_module(natural DEPENDENCIES mp-units::isq HEADERS include/units/natural/natural.h)
+add_units_module(natural DEPENDENCIES mp-units::isq HEADERS include/units/systems/natural/natural.h)
diff --git a/src/systems/natural/include/units/natural/natural.h b/src/systems/natural/include/units/systems/natural/natural.h
similarity index 95%
rename from src/systems/natural/include/units/natural/natural.h
rename to src/systems/natural/include/units/systems/natural/natural.h
index a9d22eb3..205595f3 100644
--- a/src/systems/natural/include/units/natural/natural.h
+++ b/src/systems/natural/include/units/systems/natural/natural.h
@@ -22,10 +22,10 @@
 
 #pragma once
 
-#include <units/isq/mechanics.h>
-#include <units/isq/space_and_time.h>
-#include <units/si/prefixes.h>
 #include <units/system_reference.h>
+#include <units/systems/isq/mechanics.h>
+#include <units/systems/isq/space_and_time.h>
+#include <units/systems/si/prefixes.h>
 #include <units/unit.h>
 
 namespace units::natural {
diff --git a/src/systems/si/CMakeLists.txt b/src/systems/si/CMakeLists.txt
index 4e2ed458..2e625e0d 100644
--- a/src/systems/si/CMakeLists.txt
+++ b/src/systems/si/CMakeLists.txt
@@ -24,6 +24,6 @@ cmake_minimum_required(VERSION 3.19)
 
 add_units_module(
     si DEPENDENCIES mp-units::isq
-    HEADERS include/units/si/constants.h include/units/si/prefixes.h include/units/si/si.h
-            include/units/si/unit_symbols.h include/units/si/units.h
+    HEADERS include/units/systems/si/constants.h include/units/systems/si/prefixes.h include/units/systems/si/si.h
+    include/units/systems/si/unit_symbols.h include/units/systems/si/units.h
 )
diff --git a/src/systems/si/include/units/si/constants.h b/src/systems/si/include/units/systems/si/constants.h
similarity index 98%
rename from src/systems/si/include/units/si/constants.h
rename to src/systems/si/include/units/systems/si/constants.h
index b8fe0eb2..b10ea7f7 100644
--- a/src/systems/si/include/units/si/constants.h
+++ b/src/systems/si/include/units/systems/si/constants.h
@@ -22,9 +22,9 @@
 
 #pragma once
 
-#include <units/isq/isq.h>
+#include <units/systems/isq/isq.h>
 #include <units/quantity.h>
-#include <units/si/units.h>
+#include <units/systems/si/units.h>
 
 namespace units::si {
 
diff --git a/src/systems/si/include/units/si/prefixes.h b/src/systems/si/include/units/systems/si/prefixes.h
similarity index 100%
rename from src/systems/si/include/units/si/prefixes.h
rename to src/systems/si/include/units/systems/si/prefixes.h
diff --git a/src/systems/si/include/units/si/si.h b/src/systems/si/include/units/systems/si/si.h
similarity index 87%
rename from src/systems/si/include/units/si/si.h
rename to src/systems/si/include/units/systems/si/si.h
index e97aff6d..7a267475 100644
--- a/src/systems/si/include/units/si/si.h
+++ b/src/systems/si/include/units/systems/si/si.h
@@ -22,7 +22,7 @@
 
 #pragma once
 
-#include <units/si/constants.h>
-#include <units/si/prefixes.h>
-#include <units/si/unit_symbols.h>
-#include <units/si/units.h>
+#include <units/systems/si/constants.h>
+#include <units/systems/si/prefixes.h>
+#include <units/systems/si/unit_symbols.h>
+#include <units/systems/si/units.h>
diff --git a/src/systems/si/include/units/si/unit_symbols.h b/src/systems/si/include/units/systems/si/unit_symbols.h
similarity index 99%
rename from src/systems/si/include/units/si/unit_symbols.h
rename to src/systems/si/include/units/systems/si/unit_symbols.h
index f61b12f7..f8546abb 100644
--- a/src/systems/si/include/units/si/unit_symbols.h
+++ b/src/systems/si/include/units/systems/si/unit_symbols.h
@@ -22,8 +22,8 @@
 
 #pragma once
 
-#include <units/si/prefixes.h>
-#include <units/si/units.h>
+#include <units/systems/si/prefixes.h>
+#include <units/systems/si/units.h>
 
 namespace units::si::unit_symbols {
 
diff --git a/src/systems/si/include/units/si/units.h b/src/systems/si/include/units/systems/si/units.h
similarity index 90%
rename from src/systems/si/include/units/si/units.h
rename to src/systems/si/include/units/systems/si/units.h
index cd4fcf65..b3542535 100644
--- a/src/systems/si/include/units/si/units.h
+++ b/src/systems/si/include/units/systems/si/units.h
@@ -22,8 +22,8 @@
 
 #pragma once
 
-#include <units/isq/base_quantities.h>
-#include <units/si/prefixes.h>
+#include <units/systems/isq/base_quantities.h>
+#include <units/systems/si/prefixes.h>
 #include <units/unit.h>
 
 namespace units::si {
@@ -36,6 +36,7 @@ inline constexpr struct gram : named_unit<"g", isq::mass> {} gram;
 inline constexpr struct kilogram : decltype(kilo<gram>) {} kilogram;
 inline constexpr struct ampere : named_unit<"A", isq::electric_current> {} ampere;
 inline constexpr struct kelvin : named_unit<"K", isq::thermodynamic_temperature> {} kelvin;
+// inline constexpr struct kelvin : named_unit<"K", isq::thermodynamic_temperature, not_for<isq::Celsius_temperature>> {} kelvin;
 inline constexpr struct mole : named_unit<"mol", isq::amount_of_substance> {} mole;
 inline constexpr struct candela : named_unit<"cd", isq::luminous_intensity> {} candela;
 
@@ -43,6 +44,7 @@ inline constexpr struct candela : named_unit<"cd", isq::luminous_intensity> {} c
 inline constexpr struct radian : named_unit<"rad", metre / metre> {} radian;
 inline constexpr struct steradian : named_unit<"sr", square<metre> / square<metre>> {} steradian;
 inline constexpr struct hertz : named_unit<"Hz", 1 / second> {} hertz;
+// inline constexpr struct hertz : named_unit<"Hz", 1 / second, only_for<isq::frequency>> {} hertz;
 inline constexpr struct newton : named_unit<"N", kilogram * metre / square<second>> {} newton;
 inline constexpr struct pascal : named_unit<"Pa", newton / square<metre>> {} pascal;
 inline constexpr struct joule : named_unit<"J", newton * metre> {} joule;
@@ -55,6 +57,7 @@ inline constexpr struct siemens : named_unit<"S", 1 / ohm> {} siemens;
 inline constexpr struct weber : named_unit<"Wb", volt * second> {} weber;
 inline constexpr struct tesla : named_unit<"T", weber / square<metre>> {} tesla;
 inline constexpr struct henry : named_unit<"H", weber / ampere> {} henry;
+// inline constexpr struct degree_Celsius : named_unit<basic_symbol_text{"°C", "`C"}, kelvin, offset<-mag<273150> * milli<kelvin>>, only_for<isq::Celsius_temperature>> {} degree_Celsius;
 inline constexpr struct degree_Celsius : named_unit<basic_symbol_text{"°C", "`C"}, kelvin> {} degree_Celsius;
 inline constexpr struct lumen : named_unit<"lm", candela * steradian> {} lumen;
 inline constexpr struct lux : named_unit<"lx", lumen / square<metre>> {} lux;
@@ -69,7 +72,7 @@ inline constexpr struct hour : named_unit<"h", mag<60> * minute> {} hour;
 inline constexpr struct day : named_unit<"d", mag<24> * hour> {} day;
 inline constexpr struct astronomical_unit : named_unit<"au", mag<149'597'870'700> * metre> {} astronomical_unit;
 inline constexpr struct degree : named_unit<basic_symbol_text{"°", "deg"}, mag_pi / mag<180> * radian> {} degree;
-// TODO how to disambiguate below angular units from time units
+// TODO how to disambiguate below angular units from time units (inline namespace `plane_angle`?)
 // inline constexpr struct minute : named_unit<basic_symbol_text{"′", "'"}, mag<ratio{1, 60}> * degree> {} minute;
 // inline constexpr struct second : named_unit<basic_symbol_text{"″", "''"}, mag<ratio{1, 60}> * minute> {} second;
 inline constexpr struct are : named_unit<"a", square<deca<metre>>> {} are;
diff --git a/src/systems/typographic/CMakeLists.txt b/src/systems/typographic/CMakeLists.txt
index d9c0419b..4de3b310 100644
--- a/src/systems/typographic/CMakeLists.txt
+++ b/src/systems/typographic/CMakeLists.txt
@@ -23,5 +23,5 @@
 cmake_minimum_required(VERSION 3.19)
 
 add_units_module(
-    typographic DEPENDENCIES mp-units::usc HEADERS include/units/typographic/typographic.h
+    typographic DEPENDENCIES mp-units::usc HEADERS include/units/systems/typographic/typographic.h
 )
diff --git a/src/systems/typographic/include/units/typographic/typographic.h b/src/systems/typographic/include/units/systems/typographic/typographic.h
similarity index 98%
rename from src/systems/typographic/include/units/typographic/typographic.h
rename to src/systems/typographic/include/units/systems/typographic/typographic.h
index 408bb49a..c5c6cf5b 100644
--- a/src/systems/typographic/include/units/typographic/typographic.h
+++ b/src/systems/typographic/include/units/systems/typographic/typographic.h
@@ -22,8 +22,8 @@
 
 #pragma once
 
+#include <units/systems/usc/usc.h>
 #include <units/unit.h>
-#include <units/usc/usc.h>
 
 namespace units::typographic {
 
diff --git a/src/systems/usc/CMakeLists.txt b/src/systems/usc/CMakeLists.txt
index fb8c4040..9428fa16 100644
--- a/src/systems/usc/CMakeLists.txt
+++ b/src/systems/usc/CMakeLists.txt
@@ -22,4 +22,4 @@
 
 cmake_minimum_required(VERSION 3.19)
 
-add_units_module(usc DEPENDENCIES mp-units::si mp-units::international HEADERS include/units/usc/usc.h)
+add_units_module(usc DEPENDENCIES mp-units::si mp-units::international HEADERS include/units/systems/usc/usc.h)
diff --git a/src/systems/usc/include/units/usc/usc.h b/src/systems/usc/include/units/systems/usc/usc.h
similarity index 99%
rename from src/systems/usc/include/units/usc/usc.h
rename to src/systems/usc/include/units/systems/usc/usc.h
index 675fba56..b3699d38 100644
--- a/src/systems/usc/include/units/usc/usc.h
+++ b/src/systems/usc/include/units/systems/usc/usc.h
@@ -22,7 +22,7 @@
 
 #pragma once
 
-#include <units/international/international.h>
+#include <units/systems/international/international.h>
 #include <units/unit.h>
 
 namespace units::usc {
diff --git a/test/unit_test/runtime/distribution_test.cpp b/test/unit_test/runtime/distribution_test.cpp
index 7099f3e1..412ce861 100644
--- a/test/unit_test/runtime/distribution_test.cpp
+++ b/test/unit_test/runtime/distribution_test.cpp
@@ -23,8 +23,8 @@
 #include <catch2/catch_test_macros.hpp>
 #include <units/quantity_io.h>
 #include <units/random.h>
-#include <units/si/unit_symbols.h>
-#include <units/si/units.h>
+#include <units/systems/si/unit_symbols.h>
+#include <units/systems/si/units.h>
 #include <array>
 #include <initializer_list>
 #include <random>
diff --git a/test/unit_test/runtime/fmt_test.cpp b/test/unit_test/runtime/fmt_test.cpp
index 17a72bd8..682a0a84 100644
--- a/test/unit_test/runtime/fmt_test.cpp
+++ b/test/unit_test/runtime/fmt_test.cpp
@@ -22,13 +22,13 @@
 
 #include <catch2/catch_test_macros.hpp>
 #include <catch2/matchers/catch_matchers_exception.hpp>
-#include <units/cgs/cgs.h>
 #include <units/customization_points.h>
 #include <units/format.h>
-#include <units/isq/mechanics.h>
-#include <units/isq/space_and_time.h>
 #include <units/quantity_io.h>
-#include <units/si/si.h>
+#include <units/systems/cgs/cgs.h>
+#include <units/systems/isq/mechanics.h>
+#include <units/systems/isq/space_and_time.h>
+#include <units/systems/si/si.h>
 #include <iomanip>
 #include <limits>
 #include <locale>
diff --git a/test/unit_test/runtime/math_test.cpp b/test/unit_test/runtime/math_test.cpp
index 6b0c016b..f1420cb1 100644
--- a/test/unit_test/runtime/math_test.cpp
+++ b/test/unit_test/runtime/math_test.cpp
@@ -22,12 +22,12 @@
 
 #include "almost_equals.h"
 #include <catch2/catch_all.hpp>
-#include <units/angular/angular.h>
-#include <units/isq/space_and_time.h>
 #include <units/math.h>
 #include <units/quantity_io.h>
-#include <units/si/unit_symbols.h>
-#include <units/si/units.h>
+#include <units/systems/angular/angular.h>
+#include <units/systems/isq/space_and_time.h>
+#include <units/systems/si/unit_symbols.h>
+#include <units/systems/si/units.h>
 #include <limits>
 
 using namespace units;
diff --git a/test/unit_test/static/hep_test.cpp b/test/unit_test/static/hep_test.cpp
index 2909d801..cbe016e1 100644
--- a/test/unit_test/static/hep_test.cpp
+++ b/test/unit_test/static/hep_test.cpp
@@ -20,10 +20,10 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
-#include <units/hep/hep.h>
-#include <units/isq/mechanics.h>
-#include <units/isq/space_and_time.h>
-#include <units/si/si.h>
+#include <units/systems/hep/hep.h>
+#include <units/systems/isq/mechanics.h>
+#include <units/systems/isq/space_and_time.h>
+#include <units/systems/si/si.h>
 
 template<class T>
   requires units::is_scalar<T>
diff --git a/test/unit_test/static/iec80000_test.cpp b/test/unit_test/static/iec80000_test.cpp
index 9a386a41..ce35b3cb 100644
--- a/test/unit_test/static/iec80000_test.cpp
+++ b/test/unit_test/static/iec80000_test.cpp
@@ -20,8 +20,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
-#include <units/iec80000/iec80000.h>
-#include <units/si/unit_symbols.h>
+#include <units/systems/iec80000/iec80000.h>
+#include <units/systems/si/unit_symbols.h>
 
 /* ************** DERIVED DIMENSIONS THAT INCLUDE UNITS WITH SPECIAL NAMES **************** */
 
diff --git a/test/unit_test/static/imperial_test.cpp b/test/unit_test/static/imperial_test.cpp
index eefd753b..19bcabe8 100644
--- a/test/unit_test/static/imperial_test.cpp
+++ b/test/unit_test/static/imperial_test.cpp
@@ -20,9 +20,9 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
-#include <units/imperial/imperial.h>
-#include <units/isq/space_and_time.h>
-#include <units/si/unit_symbols.h>
+#include <units/systems/imperial/imperial.h>
+#include <units/systems/isq/space_and_time.h>
+#include <units/systems/si/unit_symbols.h>
 
 namespace {
 
diff --git a/test/unit_test/static/isq_angle_test.cpp b/test/unit_test/static/isq_angle_test.cpp
index 90fce27a..c51389f7 100644
--- a/test/unit_test/static/isq_angle_test.cpp
+++ b/test/unit_test/static/isq_angle_test.cpp
@@ -20,8 +20,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
-#include <units/isq_angle/isq_angle.h>
-#include <units/si/unit_symbols.h>
+#include <units/systems/isq_angle/isq_angle.h>
+#include <units/systems/si/unit_symbols.h>
 
 namespace {
 
diff --git a/test/unit_test/static/isq_test.cpp b/test/unit_test/static/isq_test.cpp
index 3a58acbb..73fb3e25 100644
--- a/test/unit_test/static/isq_test.cpp
+++ b/test/unit_test/static/isq_test.cpp
@@ -20,8 +20,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
-#include <units/isq/isq.h>
-#include <units/si/unit_symbols.h>
+#include <units/systems/isq/isq.h>
+#include <units/systems/si/unit_symbols.h>
 
 namespace {
 
diff --git a/test/unit_test/static/reference_test.cpp b/test/unit_test/static/reference_test.cpp
index c2a3b0c7..c540671e 100644
--- a/test/unit_test/static/reference_test.cpp
+++ b/test/unit_test/static/reference_test.cpp
@@ -25,8 +25,8 @@
 #include <units/quantity.h>
 #include <units/quantity_spec.h>
 #include <units/reference.h>
-#include <units/si/prefixes.h>
 #include <units/system_reference.h>
+#include <units/systems/si/prefixes.h>
 #include <units/unit.h>
 
 namespace {
diff --git a/test/unit_test/static/unit_test.cpp b/test/unit_test/static/unit_test.cpp
index f5bc7b9e..fafe0929 100644
--- a/test/unit_test/static/unit_test.cpp
+++ b/test/unit_test/static/unit_test.cpp
@@ -23,7 +23,7 @@
 #include "test_tools.h"
 #include <units/dimension.h>
 #include <units/reference.h>
-#include <units/si/prefixes.h>
+#include <units/systems/si/prefixes.h>
 #include <units/unit.h>
 
 namespace {
diff --git a/test/unit_test/static/usc_test.cpp b/test/unit_test/static/usc_test.cpp
index fe2d9582..5b7929e1 100644
--- a/test/unit_test/static/usc_test.cpp
+++ b/test/unit_test/static/usc_test.cpp
@@ -20,9 +20,9 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
-#include <units/isq/space_and_time.h>
-#include <units/si/unit_symbols.h>
-#include <units/usc/usc.h>
+#include <units/systems/isq/space_and_time.h>
+#include <units/systems/si/unit_symbols.h>
+#include <units/systems/usc/usc.h>
 
 namespace {