diff --git a/src/tools/std20/type_traits.hpp b/src/tools/std20/type_traits.hpp
index 6bbcb71..f27eda1 100644
--- a/src/tools/std20/type_traits.hpp
+++ b/src/tools/std20/type_traits.hpp
@@ -10,6 +10,8 @@ namespace njoy {
 namespace tools {
 namespace std20 {
 
+// remove_cvref
+
 template < typename T >
 struct remove_cvref {
   using type = std::remove_cv_t< std::remove_reference_t< T > >;
@@ -18,6 +20,8 @@ struct remove_cvref {
 template < typename T >
 using remove_cvref_t = typename remove_cvref< T >::type;
 
+// type_identity
+
 template < typename T >
 struct type_identity {
   using type = T;
@@ -26,6 +30,47 @@ struct type_identity {
 template < typename T >
 using type_identity_t = typename type_identity< T >::type;
 
+// basic_common_reference
+
+template < typename T, typename U, template < typename > typename TQual,
+           template < typename > typename UQual>
+struct basic_common_reference {};
+
+// common_reference
+
+template < typename... >
+struct common_reference;
+
+template < typename... Ts >
+using common_reference_t = typename common_reference< Ts... >::type;
+
+#include "tools/std20/type_traits/details/common_reference.hpp"
+
+template <>
+struct common_reference<> {};
+
+template < typename T >
+struct common_reference< T > {
+
+    using type = T;
+};
+
+template < typename T1, typename T2 >
+struct common_reference< T1, T2 > : detail::binary_common_ref<T1, T2> {};
+
+template < typename T1, typename T2, typename... Rest >
+struct common_reference< T1, T2, Rest... >
+  : detail::multiple_common_reference< void, T1, T2, Rest... > {};
+
+// is_bounded_array
+// is_unbounded_array
+// is_nothrow_convertible
+// is_layout_compatible
+// is_pointer_interconvertible_base_of
+// is_pointer_interconvertible_with_class
+// is_corresponding_member
+// is_constant_evaluated
+
 } // std20 namespace
 } // tools namespace
 } // njoy namespace
diff --git a/src/tools/std20/type_traits/details/common_reference.hpp b/src/tools/std20/type_traits/details/common_reference.hpp
new file mode 100644
index 0000000..08a5006
--- /dev/null
+++ b/src/tools/std20/type_traits/details/common_reference.hpp
@@ -0,0 +1,187 @@
+namespace detail {
+
+template < typename T, typename U >
+struct copy_cv {
+  using type = U;
+};
+
+template < typename T, typename U >
+struct copy_cv< const T, U > {
+  using type = std::add_const_t< U >;
+};
+
+template < typename T, typename U>
+struct copy_cv<volatile T, U> {
+  using type = std::add_volatile_t< U >;
+};
+
+template < typename T, typename U >
+struct copy_cv< const volatile T, U > {
+  using type = std::add_cv_t< U >;
+};
+
+template < typename T, typename U >
+using copy_cv_t = typename copy_cv< T, U >::type;
+
+template < typename T, typename U >
+using cond_res_t = decltype( false ? std::declval< T (&)()>()()
+                                   : std::declval< U (&)()>()() );
+
+template < typename A, typename B,
+           typename X = std::remove_reference_t< A >,
+           typename Y = std::remove_reference_t< B >,
+           typename = void >
+struct common_ref {};
+
+template < typename A, typename B >
+using common_ref_t = typename common_ref< A, B >::type;
+
+template < typename A, typename B,
+           typename X = std::remove_reference_t< A >,
+           typename Y = std::remove_reference_t< B >,
+           typename = void >
+struct lval_common_ref {};
+
+template < typename A, typename B, typename X, typename Y >
+struct lval_common_ref<
+         A, B, X, Y,
+         std::enable_if_t<
+           std::is_reference_v< cond_res_t< copy_cv_t< X, Y >&,
+                                            copy_cv_t< Y, X >& > > > > {
+
+  using type = cond_res_t<copy_cv_t<X, Y>&, copy_cv_t<Y, X>&>;
+};
+
+template < typename A, typename B >
+using lval_common_ref_t = typename lval_common_ref< A, B >::type;
+
+template < typename A, typename B, typename X, typename Y >
+struct common_ref< A&, B&, X, Y> : lval_common_ref< A&, B& > {};
+
+template < typename X, typename Y >
+using rref_cr_helper_t = std::remove_reference_t< lval_common_ref_t< X&, Y& > >&&;
+
+template < typename A, typename B, typename X, typename Y >
+struct common_ref<
+         A&&, B&&, X, Y,
+         std::enable_if_t<
+           std::is_convertible_v< A&&, rref_cr_helper_t< X, Y > > &&
+           std::is_convertible_v< B&&, rref_cr_helper_t< X, Y > > > > {
+
+  using type = rref_cr_helper_t< X, Y >;
+};
+
+template < typename A, typename B, typename X, typename Y >
+struct common_ref<
+         A&&, B&, X, Y,
+         std::enable_if_t<
+           std::is_convertible_v< A&&, lval_common_ref_t< const X&, Y& > > > > {
+
+  using type = lval_common_ref_t<const X&, Y&>;
+};
+
+template < typename A, typename B, typename X, typename Y >
+struct common_ref< A&, B&&, X, Y > : common_ref< B&&, A& > {};
+
+template < typename >
+struct xref {
+
+  template <typename U> using type = U;
+};
+
+template < typename A >
+struct xref< A& > {
+
+  template < typename U >
+  using type = std::add_lvalue_reference_t< typename xref< A >::template type< U > >;
+};
+
+template < typename A >
+struct xref< A&& > {
+
+  template < typename U >
+  using type = std::add_rvalue_reference_t< typename xref< A >::template type< U > >;
+};
+
+template < typename A >
+struct xref< const A > {
+
+  template < typename U >
+  using type = std::add_const_t< typename xref< A >::template type< U > >;
+};
+
+template < typename A >
+struct xref< volatile A > {
+
+  template < typename U >
+  using type = std::add_volatile_t< typename xref< A >::template type< U > >;
+};
+
+template < typename A >
+struct xref< const volatile A > {
+
+  template < typename U >
+  using type = std::add_cv_t< typename xref< A >::template type< U > >;
+};
+
+template < template < typename... > typename AliasT, typename... Args >
+auto exists_helper( long ) -> std::false_type;
+
+template < template < typename...> typename AliasT, typename... Args,
+           typename = AliasT< Args... > >
+auto exists_helper(int) -> std::true_type;
+
+template < template < typename... > typename AliasT, typename... Args >
+inline constexpr bool exists_v = decltype( exists_helper< AliasT, Args... >(0) )::value;
+
+template < typename T, typename U >
+inline constexpr bool has_common_ref_v = exists_v< common_ref_t, T, U >;
+
+template <typename T, typename U>
+using basic_common_ref_t =
+    typename basic_common_reference< remove_cvref_t< T >,
+                                     remove_cvref_t< U >,
+                                     detail::xref< T >::template type,
+                                     detail::xref< U >::template type >::type;
+
+template < typename T, typename U >
+inline constexpr bool has_basic_common_ref_v = exists_v< basic_common_ref_t, T, U >;
+
+template < typename T, typename U >
+inline constexpr bool has_cond_res_v = exists_v< cond_res_t, T, U >;
+
+template < typename T, typename U, typename = void >
+struct binary_common_ref : std::common_type< T, U > {};
+
+template < typename T, typename U >
+struct binary_common_ref< T, U, std::enable_if_t< has_common_ref_v< T, U > > >
+  : common_ref< T, U > {};
+
+template < typename T, typename U >
+struct binary_common_ref<
+         T, U,
+         std::enable_if_t< has_basic_common_ref_v< T, U > &&
+                           ! has_common_ref_v< T, U > > > {
+
+  using type = basic_common_ref_t< T, U >;
+};
+
+template < typename T, typename U >
+struct binary_common_ref<
+         T, U,
+         std::enable_if_t< has_cond_res_v< T, U > &&
+         ! has_basic_common_ref_v< T, U > &&
+         ! has_common_ref_v< T, U > > > {
+
+  using type = cond_res_t< T, U >;
+};
+
+template < typename, typename T1, typename T2, typename... Rest >
+struct multiple_common_reference {};
+
+template < typename T1, typename T2, typename... Rest >
+struct multiple_common_reference<
+         std::void_t< common_reference_t< T1, T2 > >,
+         T1, T2, Rest... > : common_reference< common_reference_t< T1, T2 >, Rest... > {};
+
+}
diff --git a/src/tools/std20/type_traits/test/CMakeLists.txt b/src/tools/std20/type_traits/test/CMakeLists.txt
index ee75f44..e0c77fe 100644
--- a/src/tools/std20/type_traits/test/CMakeLists.txt
+++ b/src/tools/std20/type_traits/test/CMakeLists.txt
@@ -1,2 +1,3 @@
-add_cpp_test( std20.type_traits.remove_cvref remove_cvref.test.cpp )
-add_cpp_test( std20.type_traits.type_identity type_identity.test.cpp )
+add_cpp_test( std20.type_traits.remove_cvref     remove_cvref.test.cpp )
+add_cpp_test( std20.type_traits.type_identity    type_identity.test.cpp )
+add_cpp_test( std20.type_traits.common_reference common_reference.test.cpp )
diff --git a/src/tools/std20/type_traits/test/common_reference.test.cpp b/src/tools/std20/type_traits/test/common_reference.test.cpp
new file mode 100644
index 0000000..2724ace
--- /dev/null
+++ b/src/tools/std20/type_traits/test/common_reference.test.cpp
@@ -0,0 +1,87 @@
+// include Catch2
+#include <catch2/catch_test_macros.hpp>
+
+// what we are testing
+#include "tools/std20/type_traits.hpp"
+
+// other includes
+
+// convenience typedefs
+using namespace njoy::tools;
+
+class Base {};
+class First : public Base {};
+class Second : public Base {};
+
+SCENARIO( "common_reference and common_reference_t" ) {
+
+  CHECK( std::is_same_v< std20::common_reference< int >::type, int > );
+  CHECK( std::is_same_v< std20::common_reference< int&, int& >::type, int& > );
+  CHECK( std::is_same_v< std20::common_reference< const int&, int& >::type, const int& > );
+  CHECK( std::is_same_v< std20::common_reference< int&, const int& >::type, const int& > );
+  CHECK( std::is_same_v< std20::common_reference< int&&, int& >::type, const int& > );
+  CHECK( std::is_same_v< std20::common_reference< int&&, const int& >::type, const int& > );
+  CHECK( std::is_same_v< std20::common_reference< int&, int&& >::type, const int& > );
+  CHECK( std::is_same_v< std20::common_reference< const int&, int&& >::type, const int& > );
+  CHECK( std::is_same_v< std20::common_reference< int&&, int&& >::type, int&& > );
+  CHECK( std::is_same_v< std20::common_reference< const int&&, int&& >::type, const int&& > );
+  CHECK( std::is_same_v< std20::common_reference< int&&, const int&& >::type, const int&& > );
+
+  CHECK( std::is_same_v< std20::common_reference< volatile int >::type, volatile int > );
+  CHECK( std::is_same_v< std20::common_reference< volatile int&, int& >::type, volatile int& > );
+  CHECK( std::is_same_v< std20::common_reference< volatile int&, const int& >::type, const volatile int& > );
+  CHECK( std::is_same_v< std20::common_reference< const volatile int&, int& >::type, const volatile int& > );
+  CHECK( std::is_same_v< std20::common_reference< int&, volatile int& >::type, volatile int& > );
+  CHECK( std::is_same_v< std20::common_reference< const int&, volatile int& >::type, const volatile int& > );
+  CHECK( std::is_same_v< std20::common_reference< int&, const volatile int& >::type, const volatile int& > );
+
+  CHECK( std::is_same_v< std20::common_reference< Base& >::type, Base& > );
+  CHECK( std::is_same_v< std20::common_reference< Base&, First& >::type, Base& > );
+  CHECK( std::is_same_v< std20::common_reference< Base&, Second& >::type, Base& > );
+  CHECK( std::is_same_v< std20::common_reference< Base&, First&, Second& >::type, Base& > );
+  CHECK( std::is_same_v< std20::common_reference< Base&&, First&& >::type, Base&& > );
+  CHECK( std::is_same_v< std20::common_reference< Base&&, Second&& >::type, Base&& > );
+  CHECK( std::is_same_v< std20::common_reference< Base&&, First&&, Second&& >::type, Base&& > );
+  CHECK( std::is_same_v< std20::common_reference< Base&, First&& >::type, const Base& > );
+  CHECK( std::is_same_v< std20::common_reference< Base&&, First& >::type, const Base& > );
+  CHECK( std::is_same_v< std20::common_reference< Base&, First&, Second& >::type, Base& > );
+  CHECK( std::is_same_v< std20::common_reference< Base&&, First&& >::type, Base&& > );
+  CHECK( std::is_same_v< std20::common_reference< Base&&, Second&& >::type, Base&& > );
+  CHECK( std::is_same_v< std20::common_reference< Base&&, First&&, Second&& >::type, Base&& > );
+  CHECK( std::is_same_v< std20::common_reference< Base&, First&&, Second&& >::type, const Base& > );
+
+  CHECK( std::is_same_v< std20::common_reference_t< int >, int > );
+  CHECK( std::is_same_v< std20::common_reference_t< int&, int& >, int& > );
+  CHECK( std::is_same_v< std20::common_reference_t< const int&, int& >, const int& > );
+  CHECK( std::is_same_v< std20::common_reference_t< int&, const int& >, const int& > );
+  CHECK( std::is_same_v< std20::common_reference_t< int&&, int& >, const int& > );
+  CHECK( std::is_same_v< std20::common_reference_t< int&&, const int& >, const int& > );
+  CHECK( std::is_same_v< std20::common_reference_t< int&, int&& >, const int& > );
+  CHECK( std::is_same_v< std20::common_reference_t< const int&, int&& >, const int& > );
+  CHECK( std::is_same_v< std20::common_reference_t< int&&, int&& >, int&& > );
+  CHECK( std::is_same_v< std20::common_reference_t< const int&&, int&& >, const int&& > );
+  CHECK( std::is_same_v< std20::common_reference_t< int&&, const int&& >, const int&& > );
+
+  CHECK( std::is_same_v< std20::common_reference_t< volatile int >, volatile int > );
+  CHECK( std::is_same_v< std20::common_reference_t< volatile int&, int& >, volatile int& > );
+  CHECK( std::is_same_v< std20::common_reference_t< volatile int&, const int& >, const volatile int& > );
+  CHECK( std::is_same_v< std20::common_reference_t< const volatile int&, int& >, const volatile int& > );
+  CHECK( std::is_same_v< std20::common_reference_t< int&, volatile int& >, volatile int& > );
+  CHECK( std::is_same_v< std20::common_reference_t< const int&, volatile int& >, const volatile int& > );
+  CHECK( std::is_same_v< std20::common_reference_t< int&, const volatile int& >, const volatile int& > );
+
+  CHECK( std::is_same_v< std20::common_reference_t< Base& >, Base& > );
+  CHECK( std::is_same_v< std20::common_reference_t< Base&, First& >, Base& > );
+  CHECK( std::is_same_v< std20::common_reference_t< Base&, Second& >, Base& > );
+  CHECK( std::is_same_v< std20::common_reference_t< Base&, First&, Second& >, Base& > );
+  CHECK( std::is_same_v< std20::common_reference_t< Base&&, First&& >, Base&& > );
+  CHECK( std::is_same_v< std20::common_reference_t< Base&&, Second&& >, Base&& > );
+  CHECK( std::is_same_v< std20::common_reference_t< Base&&, First&&, Second&& >, Base&& > );
+  CHECK( std::is_same_v< std20::common_reference_t< Base&, First&& >, const Base& > );
+  CHECK( std::is_same_v< std20::common_reference_t< Base&&, First& >, const Base& > );
+  CHECK( std::is_same_v< std20::common_reference_t< Base&, First&, Second& >, Base& > );
+  CHECK( std::is_same_v< std20::common_reference_t< Base&&, First&& >, Base&& > );
+  CHECK( std::is_same_v< std20::common_reference_t< Base&&, Second&& >, Base&& > );
+  CHECK( std::is_same_v< std20::common_reference_t< Base&&, First&&, Second&& >, Base&& > );
+  CHECK( std::is_same_v< std20::common_reference_t< Base&, First&&, Second&& >, const Base& > );
+} // SCENARIO