diff --git a/example/measurement.cpp b/example/measurement.cpp
index db7163a5..3ef98288 100644
--- a/example/measurement.cpp
+++ b/example/measurement.cpp
@@ -21,6 +21,7 @@
 // SOFTWARE.
 
 #include <units/physical/si/acceleration.h>
+#include <cmath>
 #include <iostream>
 
 namespace {
diff --git a/src/include/units/base_dimension.h b/src/include/units/base_dimension.h
index bdf19e77..0f1e5309 100644
--- a/src/include/units/base_dimension.h
+++ b/src/include/units/base_dimension.h
@@ -23,7 +23,7 @@
 #pragma once
 
 #include <units/bits/external/fixed_string.h>
-#include <units/unit_concept.h>
+#include <units/concepts.h>
 #include <type_traits>
 
 namespace units {
@@ -54,29 +54,6 @@ struct base_dimension {
   using base_unit = U;
 };
 
-// BaseDimension
-namespace detail {
-
-#if __GNUC__ == 9 && __GNUC_MINOR__ < 2
-
-template<typename T>
-inline constexpr bool is_base_dimension = true;
-
-#else
-
-template<typename T>
-inline constexpr bool is_base_dimension = false;
-
-template<basic_fixed_string Name, typename... Params>
-inline constexpr bool is_base_dimension<base_dimension<Name, Params...>> = true;
-
-#endif
-
-}  // namespace detail
-
-template<typename T>
-concept BaseDimension = detail::is_base_dimension<typename T::base_type_workaround>;
-
 // base_dimension_less
 // TODO Remove the below when https://bugs.llvm.org/show_bug.cgi?id=32208 is fixed
 // clang-format off
diff --git a/src/include/units/bits/common_quantity.h b/src/include/units/bits/common_quantity.h
new file mode 100644
index 00000000..0a1b4da0
--- /dev/null
+++ b/src/include/units/bits/common_quantity.h
@@ -0,0 +1,66 @@
+// 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/bits/dimension_op.h>
+
+namespace units {
+
+template<Dimension D, UnitOf<D> U, Scalar Rep>
+class quantity;
+
+namespace detail {
+
+template<typename Q1, typename Q2, typename Rep>
+struct common_quantity_impl;
+
+template<typename D, typename U, typename Rep1, typename Rep2, typename Rep>
+struct common_quantity_impl<quantity<D, U, Rep1>, quantity<D, U, Rep2>, Rep> {
+  using type = quantity<D, U, Rep>;
+};
+
+template<typename D, typename U1, typename Rep1, typename U2, typename Rep2, typename Rep>
+struct common_quantity_impl<quantity<D, U1, Rep1>, quantity<D, U2, Rep2>, Rep> {
+  using type = quantity<D, downcast_unit<D, common_ratio<typename U1::ratio, typename U2::ratio>>, Rep>;
+};
+
+template<typename D1, typename U1, typename Rep1, typename D2, typename U2, typename Rep2, typename Rep>
+  requires same_unit_reference<dimension_unit<D1>, dimension_unit<D2>>::value
+struct common_quantity_impl<quantity<D1, U1, Rep1>, quantity<D2, U2, Rep2>, Rep> {
+  using type = quantity<D1, downcast_unit<D1, common_ratio<typename U1::ratio, typename U2::ratio>>, Rep>;
+};
+
+template<typename D1, typename U1, typename Rep1, typename D2, typename U2, typename Rep2, typename Rep>
+struct common_quantity_impl<quantity<D1, U1, Rep1>, quantity<D2, U2, Rep2>, Rep> {
+  using ratio1 = ratio_multiply<typename D1::base_units_ratio, typename U1::ratio>;
+  using ratio2 = ratio_multiply<typename D2::base_units_ratio, typename U2::ratio>;
+  using type = quantity<D1, downcast_unit<D1, common_ratio<ratio1, ratio2>>, Rep>;
+};
+
+}  // namespace detail
+
+template<Quantity Q1, Quantity Q2, Scalar Rep = std::common_type_t<typename Q1::rep, typename Q2::rep>>
+  requires equivalent_dim<typename Q1::dimension, typename Q2::dimension>
+using common_quantity = detail::common_quantity_impl<Q1, Q2, Rep>::type;
+
+}  // namespace units
diff --git a/src/include/units/bits/concepts.h b/src/include/units/bits/concepts.h
deleted file mode 100644
index b43d4524..00000000
--- a/src/include/units/bits/concepts.h
+++ /dev/null
@@ -1,53 +0,0 @@
-// 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/bits/external/hacks.h>
-#include <units/bits/external/numeric_concepts.h>
-#include <units/customization_points.h>
-#include <units/ratio.h>
-
-namespace units {
-
-  namespace detail {
-
-    template<typename T, typename U = T>
-    concept basic_arithmetic = // exposition only
-      std::magma<std::ranges::plus, T, U> &&
-      std::magma<std::ranges::minus, T, U> &&
-      std::magma<std::ranges::times, T, U> &&
-      std::magma<std::ranges::divided_by, T, U>;
-
-    template<typename From, typename To>
-    concept safe_convertible = // exposition only
-      std::convertible_to<From, To> &&
-      (treat_as_floating_point<To> || (!treat_as_floating_point<From>));
-
-    template<typename Rep, typename unit_from, typename unit_to>
-    concept safe_divisible = // exposition only
-      treat_as_floating_point<Rep> ||
-      ratio_divide<typename unit_from::ratio, typename unit_to::ratio>::den == 1;
-
-  }
-
-}  // namespace units
diff --git a/src/include/units/bits/deduced_unit.h b/src/include/units/bits/deduced_unit.h
index 6f170aa5..7d846b45 100644
--- a/src/include/units/bits/deduced_unit.h
+++ b/src/include/units/bits/deduced_unit.h
@@ -22,7 +22,6 @@
 
 #pragma once
 
-#include <units/unit_of_concept.h>
 #include <units/derived_dimension.h>
 
 namespace units::detail {
diff --git a/src/include/units/concepts.h b/src/include/units/concepts.h
new file mode 100644
index 00000000..9f6184f6
--- /dev/null
+++ b/src/include/units/concepts.h
@@ -0,0 +1,181 @@
+// 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/bits/external/downcasting.h>
+#include <units/bits/external/fixed_string.h>
+#include <units/bits/external/hacks.h>
+#include <units/bits/external/numeric_concepts.h>
+#include <units/bits/external/type_traits.h>
+#include <units/customization_points.h>
+
+namespace units {
+
+namespace detail {
+
+template<typename T, typename U = T>
+concept basic_arithmetic = // exposition only
+    std::magma<std::ranges::plus, T, U> &&
+    std::magma<std::ranges::minus, T, U> &&
+    std::magma<std::ranges::times, T, U> &&
+    std::magma<std::ranges::divided_by, T, U>;
+
+} // namespace detail
+
+// PrefixType
+struct prefix_type;
+
+template<typename T>
+concept PrefixType = std::derived_from<T, prefix_type>;
+
+// Prefix
+// TODO gcc:92150
+// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92150
+// namespace detail {
+
+//   template<typename T>
+//   inline constexpr bool is_prefix = false;
+
+//   template<typename PrefixType, Ratio R, basic_fixed_string Symbol>
+//   inline constexpr bool is_prefix<prefix<PrefixType, R, Symbol>> = true;
+
+// }  // namespace detail
+
+template<typename T>
+//  concept Prefix = detail::is_prefix<T>;
+concept Prefix = true;
+
+
+namespace detail {
+
+template<typename T>
+inline constexpr bool is_ratio = false;
+
+} // namespace detail
+
+template<typename T>
+concept Ratio = detail::is_ratio<T>;
+
+
+// UnitRatio
+template<typename R>
+concept UnitRatio = Ratio<R> && (R::num * R::den > 0);
+
+// Unit
+template<typename U, UnitRatio R>
+struct scaled_unit;
+
+template<typename T>
+concept Unit = is_derived_from_instantiation<T, scaled_unit>;
+
+// BaseDimension
+template<basic_fixed_string Name, Unit U>
+struct base_dimension;
+
+namespace detail {
+
+#if __GNUC__ == 9 && __GNUC_MINOR__ < 2
+
+template<typename T>
+inline constexpr bool is_base_dimension = true;
+
+#else
+
+template<typename T>
+inline constexpr bool is_base_dimension = false;
+
+template<basic_fixed_string Name, typename... Params>
+inline constexpr bool is_base_dimension<base_dimension<Name, Params...>> = true;
+
+#endif
+
+}  // namespace detail
+
+template<typename T>
+concept BaseDimension = detail::is_base_dimension<typename T::base_type_workaround>;  // TODO Replace with is_derived_from_instantiation when fixed
+
+// Exponent
+namespace detail {
+
+template<typename T>
+inline constexpr bool is_exp = false;
+
+}  // namespace detail
+
+template<typename T>
+concept Exponent = detail::is_exp<T>;
+
+// DerivedDimension
+template<typename...>
+struct derived_dimension;
+
+template<typename T>
+concept DerivedDimension = std::is_empty_v<T> && is_instantiation<downcast_base_t<T>, derived_dimension>;
+
+// Dimension
+template<typename T>
+concept Dimension = BaseDimension<T> || DerivedDimension<T>;
+
+// UnitOf
+namespace detail {
+
+template<Dimension D>
+struct dimension_unit_impl;
+
+template<BaseDimension D>
+struct dimension_unit_impl<D> {
+  using type = D::base_unit;
+};
+
+template<DerivedDimension D>
+struct dimension_unit_impl<D> {
+  using type = D::coherent_unit;
+};
+
+} // namespace detail
+
+template<Dimension D>
+using dimension_unit = detail::dimension_unit_impl<D>::type;
+
+template<typename U, typename D>
+concept UnitOf =
+  Unit<U> &&
+  Dimension<D> &&
+  std::same_as<typename U::reference, typename dimension_unit<D>::reference>;
+
+// Quantity
+namespace detail {
+
+template<typename T>
+inline constexpr bool is_quantity = false;
+
+}  // namespace detail
+
+template<typename T>
+concept Quantity = detail::is_quantity<T>;
+
+// Scalar
+template<typename T>
+concept Scalar = (!Quantity<T>) && std::regular<T> && std::totally_ordered<T> && detail::basic_arithmetic<T>;
+
+}  // namespace units
diff --git a/src/include/units/customization_points.h b/src/include/units/customization_points.h
index 9613d554..0e8b70bd 100644
--- a/src/include/units/customization_points.h
+++ b/src/include/units/customization_points.h
@@ -22,94 +22,120 @@
 
 #pragma once
 
+#include <limits>
 #include <type_traits>
-#include <cmath>
 
 namespace units {
 
-  // treat_as_floating_point
+/**
+ * @brief Specifies if a value of a type should be treated as a floating-point value
+ * 
+ * This type trait should be specialized for a custom representation type to specify
+ * that values fo this type should be treated by the library as a floating-point ones
+ * which will enable implicit conversions between quantities.
+ * 
+ * @tparam Rep a representation type for which a type trait is defined
+ */
+template<typename Rep>
+inline constexpr bool treat_as_floating_point = std::is_floating_point_v<Rep>;
 
-  template<typename Rep>  // TODO Conceptify that
-  inline constexpr bool treat_as_floating_point = std::is_floating_point_v<Rep>;
+/**
+ * @brief A type trait that defines zero, one, min, and max for a representation type
+ * 
+ * The zero, one, min, and max member functions in units::quantity forward their work to
+ * these methods. This type can be specialized if the representation Rep requires a specific
+ * implementation to return these quantity objects.
+ * 
+ * @tparam Rep a representation type for which a type trait is defined
+ */
+template<typename Rep>
+struct quantity_values {
+  static constexpr Rep zero() noexcept { return Rep(0); }
+  static constexpr Rep one() noexcept { return Rep(1); }
+  static constexpr Rep min() noexcept { return std::numeric_limits<Rep>::lowest(); }
+  static constexpr Rep max() noexcept { return std::numeric_limits<Rep>::max(); }
+};
 
-  // // isnan
-  // namespace isnan_impl {
 
-  //   // non-ADL lookup block
-  //   void isnan(); // undefined
 
-  //   template<typename>
-  //   inline constexpr bool has_customization = false;
+// // isnan
+// namespace isnan_impl {
 
-  //   template<typename T>
-  //     requires requires(const T& t) {
-  //       { isnan(t) } -> bool;
-  //     }
-  //   inline constexpr bool has_customization<T> = true;
+//   // non-ADL lookup block
+//   void isnan(); // undefined
 
-  //   struct fn {
-  //     template<typename T>
-  //     constexpr bool operator()(const T&) const
-  //     {
-  //       return false;
-  //     }
+//   template<typename>
+//   inline constexpr bool has_customization = false;
 
-  //     template<typename T>
-  //       requires treat_as_floating_point<T>
-  //     constexpr bool operator()(const T& value) const
-  //     {
-  //       return std::isnan(value);
-  //     }
+//   template<typename T>
+//     requires requires(const T& t) {
+//       { isnan(t) } -> bool;
+//     }
+//   inline constexpr bool has_customization<T> = true;
 
-  //     template<typename T>
-  //       requires treat_as_floating_point<T> && has_customization<T>
-  //     constexpr bool operator()(const T& value) const
-  //     {
-  //       return isnan(value);  // uses ADL
-  //     }
-  //   };
-  // }
+//   struct fn {
+//     template<typename T>
+//     constexpr bool operator()(const T&) const
+//     {
+//       return false;
+//     }
 
-  // inline constexpr isnan_impl::fn isnan{};
+//     template<typename T>
+//       requires treat_as_floating_point<T>
+//     constexpr bool operator()(const T& value) const
+//     {
+//       return std::isnan(value);
+//     }
 
-  // // isfinite
-  // namespace isfinite_impl {
+//     template<typename T>
+//       requires treat_as_floating_point<T> && has_customization<T>
+//     constexpr bool operator()(const T& value) const
+//     {
+//       return isnan(value);  // uses ADL
+//     }
+//   };
+// }
 
-  //   // non-ADL lookup block
-  //   void isfinite(); // undefined
+// inline constexpr isnan_impl::fn isnan{};
 
-  //   template<typename>
-  //   inline constexpr bool has_customization = false;
+// // isfinite
+// namespace isfinite_impl {
 
-  //   template<typename T>
-  //     requires requires(const T& t) {
-  //       { isfinite(t) } -> bool;
-  //     }
-  //   inline constexpr bool has_customization<T> = true;
+//   // non-ADL lookup block
+//   void isfinite(); // undefined
 
-  //   struct fn {
-  //     template<typename T>
-  //     constexpr bool operator()(const T&) const
-  //     {
-  //       return true;
-  //     }
+//   template<typename>
+//   inline constexpr bool has_customization = false;
 
-  //     template<typename T>
-  //       requires treat_as_floating_point<T>
-  //     constexpr bool operator()(const T& value) const
-  //     {
-  //       return std::isfinite(value);
-  //     }
+//   template<typename T>
+//     requires requires(const T& t) {
+//       { isfinite(t) } -> bool;
+//     }
+//   inline constexpr bool has_customization<T> = true;
 
-  //     template<typename T>
-  //       requires treat_as_floating_point<T> && has_customization<T>
-  //     constexpr bool operator()(const T& value) const
-  //     {
-  //       return isfinite(value);  // uses ADL
-  //     }
-  //   };
-  // }
+//   struct fn {
+//     template<typename T>
+//     constexpr bool operator()(const T&) const
+//     {
+//       return true;
+//     }
 
-  // inline constexpr isfinite_impl::fn isfinite{};
+//     template<typename T>
+//       requires treat_as_floating_point<T>
+//     constexpr bool operator()(const T& value) const
+//     {
+//       return std::isfinite(value);
+//     }
 
-}
+//     template<typename T>
+//       requires treat_as_floating_point<T> && has_customization<T>
+//     constexpr bool operator()(const T& value) const
+//     {
+//       return isfinite(value);  // uses ADL
+//     }
+//   };
+// }
+
+// inline constexpr isfinite_impl::fn isfinite{};
+
+} // namespace units
diff --git a/src/include/units/derived_dimension.h b/src/include/units/derived_dimension.h
index a0846a81..73870a26 100644
--- a/src/include/units/derived_dimension.h
+++ b/src/include/units/derived_dimension.h
@@ -31,19 +31,6 @@
 
 namespace units {
 
-// Exponent
-namespace detail {
-
-template<typename T>
-inline constexpr bool is_exp = false;
-
-// partial specialization for an exp type provided below
-
-}  // namespace detail
-
-template<typename T>
-concept Exponent = detail::is_exp<T>;
-
 /**
  * @brief A derived dimension
  * 
@@ -79,14 +66,6 @@ struct derived_dimension<> : downcast_base<derived_dimension<>> {};
 template<Exponent E, Exponent... ERest>
 struct derived_dimension<E, ERest...> : downcast_base<derived_dimension<E, ERest...>> {};  // TODO rename to 'dimension'?
 
-// DerivedDimension
-template<typename T>
-concept DerivedDimension = std::is_empty_v<T> && is_instantiation<downcast_base_t<T>, derived_dimension>;
-
-// Dimension
-template<typename T>
-concept Dimension = BaseDimension<T> || DerivedDimension<T>;
-
 /**
  * @brief A power of factor corresponding to the dimension of a quantity
  *
diff --git a/src/include/units/prefix.h b/src/include/units/prefix.h
index 2117178d..eaded432 100644
--- a/src/include/units/prefix.h
+++ b/src/include/units/prefix.h
@@ -35,9 +35,6 @@ namespace units {
  */
 struct prefix_type {};
 
-template<typename T>
-concept PrefixType = std::derived_from<T, prefix_type>;
-
 /**
  * @brief No prefix possible for the unit
  * 
@@ -72,25 +69,9 @@ struct prefix_base : downcast_base<prefix_base<PT, R>> {
  * @tparam R factor to be used to scale a unit
  */
 template<typename Child, PrefixType PT, basic_fixed_string Symbol, Ratio R>
-  requires(!std::same_as<PT, no_prefix>)
+  requires (!std::same_as<PT, no_prefix>)
 struct prefix : downcast_child<Child, detail::prefix_base<PT, ratio<R::num, R::den>>> {
   static constexpr auto symbol = Symbol;
 };
 
-// TODO gcc:92150
-// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92150
-// namespace detail {
-
-//   template<typename T>
-//   inline constexpr bool is_prefix = false;
-
-//   template<typename PrefixType, Ratio R, basic_fixed_string Symbol>
-//   inline constexpr bool is_prefix<prefix<PrefixType, R, Symbol>> = true;
-
-// }  // namespace detail
-
-template<typename T>
-//  concept Prefix = detail::is_prefix<T>;
-concept Prefix = true;
-
 }  // namespace units
diff --git a/src/include/units/quantity.h b/src/include/units/quantity.h
index c87e9175..acbc276b 100644
--- a/src/include/units/quantity.h
+++ b/src/include/units/quantity.h
@@ -22,255 +22,32 @@
 
 #pragma once
 
-#include <units/bits/concepts.h>
+#include <units/bits/common_quantity.h>
 #include <units/bits/dimension_op.h>
 #include <units/bits/unit_text.h>
+#include <units/quantity_cast.h>
 
 #if __GNUC__ >= 10
 #include <compare>
 #endif
 
-#include <limits>
 #include <ostream>
 
 namespace units {
 
-// Quantity
 namespace detail {
 
-template<typename T>
-inline constexpr bool is_quantity = false;
+template<typename From, typename To>
+concept safe_convertible = // exposition only
+    std::convertible_to<From, To> &&
+    (treat_as_floating_point<To> || (!treat_as_floating_point<From>));
 
-// partial specialization below after the first quantity forward declaration
+template<typename Rep, typename UnitFrom, typename UnitTo>
+concept safe_divisible = // exposition only
+    treat_as_floating_point<Rep> ||
+    ratio_divide<typename UnitFrom::ratio, typename UnitTo::ratio>::den == 1;
 
-}  // namespace detail
-
-template<typename T>
-concept Quantity = detail::is_quantity<T>;
-
-// QuantityOf
-template<typename T, typename Dim>
-concept QuantityOf = Quantity<T> && Dimension<Dim> && equivalent_dim<typename T::dimension, Dim>;
-
-// Scalar
-template<typename T>
-concept Scalar = (!Quantity<T>) && std::regular<T> && std::totally_ordered<T> && detail::basic_arithmetic<T>;
-
-template<Dimension D, UnitOf<D> U, Scalar Rep>
-class quantity;
-
-namespace detail {
-
-template<typename D, typename U, typename Rep>
-inline constexpr bool is_quantity<quantity<D, U, Rep>> = true;
-
-}  // namespace detail
-
-// common_quantity
-namespace detail {
-
-template<typename Q1, typename Q2, typename Rep>
-struct common_quantity_impl;
-
-template<typename D, typename U, typename Rep1, typename Rep2, typename Rep>
-struct common_quantity_impl<quantity<D, U, Rep1>, quantity<D, U, Rep2>, Rep> {
-  using type = quantity<D, U, Rep>;
-};
-
-template<typename D, typename U1, typename Rep1, typename U2, typename Rep2, typename Rep>
-struct common_quantity_impl<quantity<D, U1, Rep1>, quantity<D, U2, Rep2>, Rep> {
-  using type = quantity<D, downcast_unit<D, common_ratio<typename U1::ratio, typename U2::ratio>>, Rep>;
-};
-
-template<typename D1, typename U1, typename Rep1, typename D2, typename U2, typename Rep2, typename Rep>
-  requires same_unit_reference<dimension_unit<D1>, dimension_unit<D2>>::value
-struct common_quantity_impl<quantity<D1, U1, Rep1>, quantity<D2, U2, Rep2>, Rep> {
-  using type = quantity<D1, downcast_unit<D1, common_ratio<typename U1::ratio, typename U2::ratio>>, Rep>;
-};
-
-template<typename D1, typename U1, typename Rep1, typename D2, typename U2, typename Rep2, typename Rep>
-struct common_quantity_impl<quantity<D1, U1, Rep1>, quantity<D2, U2, Rep2>, Rep> {
-  using ratio1 = ratio_multiply<typename D1::base_units_ratio, typename U1::ratio>;
-  using ratio2 = ratio_multiply<typename D2::base_units_ratio, typename U2::ratio>;
-  using type = quantity<D1, downcast_unit<D1, common_ratio<ratio1, ratio2>>, Rep>;
-};
-
-}  // namespace detail
-
-template<Quantity Q1, Quantity Q2, Scalar Rep = std::common_type_t<typename Q1::rep, typename Q2::rep>>
-  requires equivalent_dim<typename Q1::dimension, typename Q2::dimension>
-using common_quantity = detail::common_quantity_impl<Q1, Q2, Rep>::type;
-
-// quantity_cast
-namespace detail {
-
-template<typename To, typename CRatio, typename CRep, bool NumIsOne = false, bool DenIsOne = false>
-struct quantity_cast_impl {
-  template<typename Q>
-  static constexpr To cast(const Q& q)
-  {
-    if constexpr (treat_as_floating_point<CRep>) {
-      return To(static_cast<To::rep>(static_cast<CRep>(q.count()) *
-                                     (static_cast<CRep>(CRatio::num) / static_cast<CRep>(CRatio::den))));
-    } else {
-      return To(static_cast<To::rep>(static_cast<CRep>(q.count()) * static_cast<CRep>(CRatio::num) /
-                                     static_cast<CRep>(CRatio::den)));
-    }
-  }
-};
-
-template<typename To, typename CRatio, typename CRep>
-struct quantity_cast_impl<To, CRatio, CRep, true, true> {
-  template<Quantity Q>
-  static constexpr To cast(const Q& q)
-  {
-    return To(static_cast<To::rep>(q.count()));
-  }
-};
-
-template<typename To, typename CRatio, typename CRep>
-struct quantity_cast_impl<To, CRatio, CRep, true, false> {
-  template<Quantity Q>
-  static constexpr To cast(const Q& q)
-  {
-    if constexpr (treat_as_floating_point<CRep>) {
-      return To(static_cast<To::rep>(static_cast<CRep>(q.count()) * (CRep{1} / static_cast<CRep>(CRatio::den))));
-    } else {
-      return To(static_cast<To::rep>(static_cast<CRep>(q.count()) / static_cast<CRep>(CRatio::den)));
-    }
-  }
-};
-
-template<typename To, typename CRatio, typename CRep>
-struct quantity_cast_impl<To, CRatio, CRep, false, true> {
-  template<Quantity Q>
-  static constexpr To cast(const Q& q)
-  {
-    return To(static_cast<To::rep>(static_cast<CRep>(q.count()) * static_cast<CRep>(CRatio::num)));
-  }
-};
-
-template<Dimension FromD, Unit FromU, Dimension ToD, Unit ToU>
-struct cast_ratio;
-
-template<BaseDimension FromD, Unit FromU, BaseDimension ToD, Unit ToU>
-struct cast_ratio<FromD, FromU, ToD, ToU> {
-  using type = ratio_divide<typename FromU::ratio, typename ToU::ratio>;
-};
-
-template<DerivedDimension FromD, Unit FromU, DerivedDimension ToD, Unit ToU>
-  requires same_unit_reference<FromU, ToU>::value
-struct cast_ratio<FromD, FromU, ToD, ToU> {
-  using type = ratio_divide<typename FromU::ratio, typename ToU::ratio>;
-};
-
-template<DerivedDimension FromD, Unit FromU, DerivedDimension ToD, Unit ToU>
-struct cast_ratio<FromD, FromU, ToD, ToU> {
-  using from_ratio = ratio_multiply<typename FromD::base_units_ratio, typename FromU::ratio>;
-  using to_ratio = ratio_multiply<typename ToD::base_units_ratio, typename ToU::ratio>;
-  using type = ratio_divide<from_ratio, to_ratio>;
-};
-
-}  // namespace detail
-
-/**
- * @brief Explcit cast of a quantity
- * 
- * Implicit conversions between quantities of different types are allowed only for "safe"
- * (i.e. non-truncating) conversion. In such cases an explicit cast have to be used.
- * 
- * This cast gets the target quantity type to cast to. For example:
- * 
- * auto q1 = units::quantity_cast<units::si::time<units::si::second>>(1ms);
- * 
- * @tparam To a target quantity type to cast to
- */
-template<Quantity To, typename D, typename U, typename Rep>
-[[nodiscard]] constexpr auto quantity_cast(const quantity<D, U, Rep>& q)
-  requires QuantityOf<To, D> &&
-           detail::basic_arithmetic<std::common_type_t<typename To::rep, Rep, intmax_t>>
-{
-  using c_ratio = detail::cast_ratio<D, U, typename To::dimension, typename To::unit>::type;
-  using c_rep = std::common_type_t<typename To::rep, Rep, intmax_t>;
-  using ret_unit = downcast_unit<typename To::dimension, typename To::unit::ratio>;
-  using ret = quantity<typename To::dimension, ret_unit, typename To::rep>;
-  using cast = detail::quantity_cast_impl<ret, c_ratio, c_rep, c_ratio::num == 1, c_ratio::den == 1>;
-  return cast::cast(q);
-}
-
-/**
- * @brief Explcit cast of a quantity
- * 
- * Implicit conversions between quantities of different types are allowed only for "safe"
- * (i.e. non-truncating) conversion. In such cases an explicit cast have to be used.
- * 
- * This cast gets only the target dimension to cast to. For example:
- * 
- * auto q1 = units::quantity_cast<units::si::acceleration>(200Gal);
- * 
- * @tparam ToD a dimension type to use for a target quantity
- */
-template<Dimension ToD, typename D, typename U, typename Rep>
-[[nodiscard]] constexpr auto quantity_cast(const quantity<D, U, Rep>& q)
-  requires equivalent_dim<ToD, D>
-{
-  return quantity_cast<quantity<ToD, U, Rep>>(q);
-}
-
-/**
- * @brief Explcit cast of a quantity
- * 
- * Implicit conversions between quantities of different types are allowed only for "safe"
- * (i.e. non-truncating) conversion. In such cases an explicit cast have to be used.
- * 
- * This cast gets only the target unit to cast to. For example:
- * 
- * auto q1 = units::quantity_cast<units::si::second>(1ms);
- * 
- * @tparam ToU a unit type to use for a target quantity
- */
-template<Unit ToU, typename D, typename U, typename Rep>
-[[nodiscard]] constexpr auto quantity_cast(const quantity<D, U, Rep>& q)
-  requires UnitOf<ToU, D>
-{
-  return quantity_cast<quantity<D, ToU, Rep>>(q);
-}
-
-/**
- * @brief Explcit cast of a quantity
- * 
- * Implicit conversions between quantities of different types are allowed only for "safe"
- * (i.e. non-truncating) conversion. In such cases an explicit cast have to be used.
- * 
- * This cast gets only representation to cast to. For example:
- * 
- * auto q1 = units::quantity_cast<int>(1ms);
- * 
- * @tparam ToRep a representation type to use for a target quantity
- */
-template<Scalar ToRep, typename D, typename U, typename Rep>
-[[nodiscard]] constexpr auto quantity_cast(const quantity<D, U, Rep>& q)
-  requires detail::basic_arithmetic<std::common_type_t<ToRep, Rep, intmax_t>>
-{
-  return quantity_cast<quantity<D, U, ToRep>>(q);
-}
-
-/**
- * @brief A type trait that defines zero, one, min, and max for a representation type
- * 
- * The zero, one, min, and max member functions in units::quantity forward their work to
- * these methods. This type can be specialized if the representation Rep requires a specific
- * implementation to return these quantity objects.
- * 
- * @tparam Rep a representation type for which a type trait is defined
- */
-template<Scalar Rep>
-struct quantity_values {
-  static constexpr Rep zero() noexcept { return Rep(0); }
-  static constexpr Rep one() noexcept { return Rep(1); }
-  static constexpr Rep min() noexcept { return std::numeric_limits<Rep>::lowest(); }
-  static constexpr Rep max() noexcept { return std::numeric_limits<Rep>::max(); }
-};
+} // namespace detail
 
 /**
  * @brief A quantity
@@ -443,46 +220,6 @@ public:
     return *this;
   }
 
-  template<typename U2, typename Rep2>
-  [[nodiscard]] friend constexpr Quantity AUTO operator+(const quantity& lhs, const quantity<D, U2, Rep2>& rhs)
-    requires detail::basic_arithmetic<Rep, Rep2>
-  {
-    using common_rep = decltype(lhs.count() + rhs.count());
-    using ret = common_quantity<quantity, quantity<D, U2, Rep2>, common_rep>;
-    return ret(ret(lhs).count() + ret(rhs).count());
-  }
-
-  template<typename U2, typename Rep2>
-  [[nodiscard]] friend constexpr Quantity AUTO operator-(const quantity& lhs, const quantity<D, U2, Rep2>& rhs)
-    requires detail::basic_arithmetic<Rep, Rep2>
-  {
-    using common_rep = decltype(lhs.count() - rhs.count());
-    using ret = common_quantity<quantity, quantity<D, U2, Rep2>, common_rep>;
-    return ret(ret(lhs).count() - ret(rhs).count());
-  }
-
-  template<Scalar Value>
-  [[nodiscard]] friend constexpr Quantity AUTO operator%(const quantity& q, const Value& v)
-    requires (!treat_as_floating_point<Rep>) &&
-            (!treat_as_floating_point<Value>) &&
-            std::magma<std::ranges::modulus, Rep, Value>
-  {
-    using common_rep = decltype(q.count() % v);
-    using ret = quantity<D, U, common_rep>;
-    return ret(q.count() % v);
-  }
-
-  template<typename U2, typename Rep2>
-  [[nodiscard]] friend constexpr Quantity AUTO operator%(const quantity& lhs, const quantity<D, U2, Rep2>& rhs)
-    requires (!treat_as_floating_point<Rep>) &&
-            (!treat_as_floating_point<Rep2>) &&
-            std::magma<std::ranges::modulus, Rep, Rep2>
-  {
-    using common_rep = decltype(lhs.count() % rhs.count());
-    using ret = common_quantity<quantity, quantity<D, U2, Rep2>, common_rep>;
-    return ret(ret(lhs).count() % ret(rhs).count());
-  }
-
 #if __GNUC__ >= 10
 
   template<typename D2, typename U2, typename Rep2>
@@ -573,7 +310,23 @@ public:
   }
 };
 
-// TODO make hidden friends when moved to gcc-10
+template<typename D, typename U1, typename Rep1, typename U2, typename Rep2>
+[[nodiscard]] constexpr Quantity AUTO operator+(const quantity<D, U1, Rep1>& lhs, const quantity<D, U2, Rep2>& rhs)
+  requires detail::basic_arithmetic<Rep1, Rep2>
+{
+  using common_rep = decltype(lhs.count() + rhs.count());
+  using ret = common_quantity<quantity<D, U1, Rep1>, quantity<D, U2, Rep2>, common_rep>;
+  return ret(ret(lhs).count() + ret(rhs).count());
+}
+
+template<typename D, typename U1, typename Rep1, typename U2, typename Rep2>
+[[nodiscard]] constexpr Quantity AUTO operator-(const quantity<D, U1, Rep1>& lhs, const quantity<D, U2, Rep2>& rhs)
+  requires detail::basic_arithmetic<Rep1, Rep2>
+{
+  using common_rep = decltype(lhs.count() - rhs.count());
+  using ret = common_quantity<quantity<D, U1, Rep1>, quantity<D, U2, Rep2>, common_rep>;
+  return ret(ret(lhs).count() - ret(rhs).count());
+}
 
 template<typename D, typename U, typename Rep, Scalar Value>
 [[nodiscard]] constexpr Quantity AUTO operator*(const quantity<D, U, Rep>& q, const Value& v)
@@ -614,7 +367,7 @@ template<typename D1, typename U1, typename Rep1, typename D2, typename U2, type
   return ret(lhs.count() * rhs.count());
 }
 
-template<typename D, Scalar Value, typename U, typename Rep>
+template<Scalar Value, typename D, typename U, typename Rep>
 [[nodiscard]] constexpr Quantity AUTO operator/(const Value& v, const quantity<D, U, Rep>& q)
   requires std::magma<std::ranges::divided_by, Value, Rep>
 {
@@ -666,4 +419,33 @@ template<typename D1, typename U1, typename Rep1, typename D2, typename U2, type
   return ret(lhs.count() / rhs.count());
 }
 
+template<typename D, typename U, typename Rep, Scalar Value>
+[[nodiscard]] constexpr Quantity AUTO operator%(const quantity<D, U, Rep>& q, const Value& v)
+  requires (!treat_as_floating_point<Rep>) &&
+           (!treat_as_floating_point<Value>) &&
+           std::magma<std::ranges::modulus, Rep, Value>
+{
+  using common_rep = decltype(q.count() % v);
+  using ret = quantity<D, U, common_rep>;
+  return ret(q.count() % v);
+}
+
+template<typename D, typename U1, typename Rep1, typename U2, typename Rep2>
+[[nodiscard]] constexpr Quantity AUTO operator%(const quantity<D, U1, Rep1>& lhs, const quantity<D, U2, Rep2>& rhs)
+  requires (!treat_as_floating_point<Rep1>) &&
+           (!treat_as_floating_point<Rep2>) &&
+           std::magma<std::ranges::modulus, Rep1, Rep2>
+{
+  using common_rep = decltype(lhs.count() % rhs.count());
+  using ret = common_quantity<quantity<D, U1, Rep1>, quantity<D, U2, Rep2>, common_rep>;
+  return ret(ret(lhs).count() % ret(rhs).count());
+}
+
+namespace detail {
+
+template<typename D, typename U, typename Rep>
+inline constexpr bool is_quantity<quantity<D, U, Rep>> = true;
+
+}  // namespace detail
+
 }  // namespace units
diff --git a/src/include/units/quantity_cast.h b/src/include/units/quantity_cast.h
new file mode 100644
index 00000000..c84548ac
--- /dev/null
+++ b/src/include/units/quantity_cast.h
@@ -0,0 +1,188 @@
+// 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/concepts.h>
+#include <units/bits/dimension_op.h>
+
+namespace units {
+
+// QuantityOf
+template<typename T, typename Dim>
+concept QuantityOf = Quantity<T> && Dimension<Dim> && equivalent_dim<typename T::dimension, Dim>;
+
+// quantity_cast
+namespace detail {
+
+template<typename To, typename CRatio, typename CRep, bool NumIsOne = false, bool DenIsOne = false>
+struct quantity_cast_impl {
+  template<typename Q>
+  static constexpr To cast(const Q& q)
+  {
+    if constexpr (treat_as_floating_point<CRep>) {
+      return To(static_cast<To::rep>(static_cast<CRep>(q.count()) *
+                                     (static_cast<CRep>(CRatio::num) / static_cast<CRep>(CRatio::den))));
+    } else {
+      return To(static_cast<To::rep>(static_cast<CRep>(q.count()) * static_cast<CRep>(CRatio::num) /
+                                     static_cast<CRep>(CRatio::den)));
+    }
+  }
+};
+
+template<typename To, typename CRatio, typename CRep>
+struct quantity_cast_impl<To, CRatio, CRep, true, true> {
+  template<Quantity Q>
+  static constexpr To cast(const Q& q)
+  {
+    return To(static_cast<To::rep>(q.count()));
+  }
+};
+
+template<typename To, typename CRatio, typename CRep>
+struct quantity_cast_impl<To, CRatio, CRep, true, false> {
+  template<Quantity Q>
+  static constexpr To cast(const Q& q)
+  {
+    if constexpr (treat_as_floating_point<CRep>) {
+      return To(static_cast<To::rep>(static_cast<CRep>(q.count()) * (CRep{1} / static_cast<CRep>(CRatio::den))));
+    } else {
+      return To(static_cast<To::rep>(static_cast<CRep>(q.count()) / static_cast<CRep>(CRatio::den)));
+    }
+  }
+};
+
+template<typename To, typename CRatio, typename CRep>
+struct quantity_cast_impl<To, CRatio, CRep, false, true> {
+  template<Quantity Q>
+  static constexpr To cast(const Q& q)
+  {
+    return To(static_cast<To::rep>(static_cast<CRep>(q.count()) * static_cast<CRep>(CRatio::num)));
+  }
+};
+
+template<Dimension FromD, Unit FromU, Dimension ToD, Unit ToU>
+struct cast_ratio;
+
+template<BaseDimension FromD, Unit FromU, BaseDimension ToD, Unit ToU>
+struct cast_ratio<FromD, FromU, ToD, ToU> {
+  using type = ratio_divide<typename FromU::ratio, typename ToU::ratio>;
+};
+
+template<DerivedDimension FromD, Unit FromU, DerivedDimension ToD, Unit ToU>
+  requires same_unit_reference<FromU, ToU>::value
+struct cast_ratio<FromD, FromU, ToD, ToU> {
+  using type = ratio_divide<typename FromU::ratio, typename ToU::ratio>;
+};
+
+template<DerivedDimension FromD, Unit FromU, DerivedDimension ToD, Unit ToU>
+struct cast_ratio<FromD, FromU, ToD, ToU> {
+  using from_ratio = ratio_multiply<typename FromD::base_units_ratio, typename FromU::ratio>;
+  using to_ratio = ratio_multiply<typename ToD::base_units_ratio, typename ToU::ratio>;
+  using type = ratio_divide<from_ratio, to_ratio>;
+};
+
+}  // namespace detail
+
+/**
+ * @brief Explcit cast of a quantity
+ * 
+ * Implicit conversions between quantities of different types are allowed only for "safe"
+ * (i.e. non-truncating) conversion. In such cases an explicit cast have to be used.
+ * 
+ * This cast gets the target quantity type to cast to. For example:
+ * 
+ * auto q1 = units::quantity_cast<units::si::time<units::si::second>>(1ms);
+ * 
+ * @tparam To a target quantity type to cast to
+ */
+template<Quantity To, typename D, typename U, typename Rep>
+[[nodiscard]] constexpr auto quantity_cast(const quantity<D, U, Rep>& q)
+  requires QuantityOf<To, D> &&
+           detail::basic_arithmetic<std::common_type_t<typename To::rep, Rep, intmax_t>>
+{
+  using c_ratio = detail::cast_ratio<D, U, typename To::dimension, typename To::unit>::type;
+  using c_rep = std::common_type_t<typename To::rep, Rep, intmax_t>;
+  using ret_unit = downcast_unit<typename To::dimension, typename To::unit::ratio>;
+  using ret = quantity<typename To::dimension, ret_unit, typename To::rep>;
+  using cast = detail::quantity_cast_impl<ret, c_ratio, c_rep, c_ratio::num == 1, c_ratio::den == 1>;
+  return cast::cast(q);
+}
+
+/**
+ * @brief Explcit cast of a quantity
+ * 
+ * Implicit conversions between quantities of different types are allowed only for "safe"
+ * (i.e. non-truncating) conversion. In such cases an explicit cast have to be used.
+ * 
+ * This cast gets only the target dimension to cast to. For example:
+ * 
+ * auto q1 = units::quantity_cast<units::si::acceleration>(200Gal);
+ * 
+ * @tparam ToD a dimension type to use for a target quantity
+ */
+template<Dimension ToD, typename D, typename U, typename Rep>
+[[nodiscard]] constexpr auto quantity_cast(const quantity<D, U, Rep>& q)
+  requires equivalent_dim<ToD, D>
+{
+  return quantity_cast<quantity<ToD, U, Rep>>(q);
+}
+
+/**
+ * @brief Explcit cast of a quantity
+ * 
+ * Implicit conversions between quantities of different types are allowed only for "safe"
+ * (i.e. non-truncating) conversion. In such cases an explicit cast have to be used.
+ * 
+ * This cast gets only the target unit to cast to. For example:
+ * 
+ * auto q1 = units::quantity_cast<units::si::second>(1ms);
+ * 
+ * @tparam ToU a unit type to use for a target quantity
+ */
+template<Unit ToU, typename D, typename U, typename Rep>
+[[nodiscard]] constexpr auto quantity_cast(const quantity<D, U, Rep>& q)
+  requires UnitOf<ToU, D>
+{
+  return quantity_cast<quantity<D, ToU, Rep>>(q);
+}
+
+/**
+ * @brief Explcit cast of a quantity
+ * 
+ * Implicit conversions between quantities of different types are allowed only for "safe"
+ * (i.e. non-truncating) conversion. In such cases an explicit cast have to be used.
+ * 
+ * This cast gets only representation to cast to. For example:
+ * 
+ * auto q1 = units::quantity_cast<int>(1ms);
+ * 
+ * @tparam ToRep a representation type to use for a target quantity
+ */
+template<Scalar ToRep, typename D, typename U, typename Rep>
+[[nodiscard]] constexpr auto quantity_cast(const quantity<D, U, Rep>& q)
+  requires detail::basic_arithmetic<std::common_type_t<ToRep, Rep, intmax_t>>
+{
+  return quantity_cast<quantity<D, U, ToRep>>(q);
+}
+
+}  // namespace units
diff --git a/src/include/units/ratio.h b/src/include/units/ratio.h
index 6e0700a0..6ad15cef 100644
--- a/src/include/units/ratio.h
+++ b/src/include/units/ratio.h
@@ -23,6 +23,7 @@
 #pragma once
 
 #include <units/bits/external/hacks.h>
+#include <units/concepts.h>
 #include <cstdint>
 #include <numeric>
 #include <type_traits>
@@ -51,21 +52,13 @@ struct ratio {
   using type = ratio<num, den>;
 };
 
-// is_ratio
-
 namespace detail {
 
-template<typename T>
-inline constexpr bool is_ratio = false;
-
 template<intmax_t Num, intmax_t Den>
 inline constexpr bool is_ratio<ratio<Num, Den>> = true;
 
 }  // namespace detail
 
-template<typename T>
-concept Ratio = detail::is_ratio<T>;
-
 
 // ratio_add
 // TODO implement ratio_add
diff --git a/src/include/units/unit.h b/src/include/units/unit.h
index f0875a95..cc7973d8 100644
--- a/src/include/units/unit.h
+++ b/src/include/units/unit.h
@@ -28,7 +28,6 @@
 #include <units/bits/external/fixed_string.h>
 #include <units/bits/external/text_tools.h>
 #include <units/bits/external/type_traits.h>
-#include <units/unit_of_concept.h>
 #include <units/derived_dimension.h>
 #include <units/prefix.h>
 #include <units/ratio.h>
diff --git a/src/include/units/unit_concept.h b/src/include/units/unit_concept.h
deleted file mode 100644
index 9e250657..00000000
--- a/src/include/units/unit_concept.h
+++ /dev/null
@@ -1,41 +0,0 @@
-// 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/bits/external/type_traits.h>
-#include <units/ratio.h>
-
-namespace units {
-
-// UnitRatio
-template<typename R>
-concept UnitRatio = Ratio<R> && (R::num * R::den > 0);
-
-template<typename U, UnitRatio R>
-struct scaled_unit;
-
-// Unit
-template<typename T>
-concept Unit = is_derived_from_instantiation<T, scaled_unit>;
-
-}  // namespace units
diff --git a/src/include/units/unit_of_concept.h b/src/include/units/unit_of_concept.h
deleted file mode 100644
index c39a4016..00000000
--- a/src/include/units/unit_of_concept.h
+++ /dev/null
@@ -1,56 +0,0 @@
-// 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/derived_dimension.h>
-
-namespace units {
-
-namespace detail {
-
-template<Dimension D>
-struct dimension_unit_impl;
-
-template<BaseDimension D>
-struct dimension_unit_impl<D> {
-  using type = D::base_unit;
-};
-
-template<DerivedDimension D>
-struct dimension_unit_impl<D> {
-  using type = D::coherent_unit;
-};
-
-} // namespace detail
-
-template<Dimension D>
-using dimension_unit = detail::dimension_unit_impl<D>::type;
-
-// UnitOf
-template<typename U, typename D>
-concept UnitOf =
-  Unit<U> &&
-  Dimension<D> &&
-  std::same_as<typename U::reference, typename dimension_unit<D>::reference>;
-
-}  // namespace units