diff --git a/src/tools/std20/concepts/test/CMakeLists.txt b/src/tools/std20/concepts/test/CMakeLists.txt
index 65b93b7..326271c 100644
--- a/src/tools/std20/concepts/test/CMakeLists.txt
+++ b/src/tools/std20/concepts/test/CMakeLists.txt
@@ -1,5 +1,8 @@
-add_cpp_test( std20.concepts.convertible_to        convertible_to.test.cpp )
-add_cpp_test( std20.concepts.derived_from          derived_from.test.cpp )
-add_cpp_test( std20.concepts.equality_comparable   equality_comparable.test.cpp )
-add_cpp_test( std20.concepts.same_as               same_as.test.cpp )
-add_cpp_test( std20.concepts.totally_ordered       totally_ordered.test.cpp )
+add_cpp_test( std20.concepts.same_as                   same_as.test.cpp )
+add_cpp_test( std20.concepts.derived_from              derived_from.test.cpp )
+add_cpp_test( std20.concepts.convertible_to            convertible_to.test.cpp )
+add_cpp_test( std20.concepts.common_reference_with     common_reference_with.test.cpp )
+add_cpp_test( std20.concepts.common_with               common_with.test.cpp )
+
+add_cpp_test( std20.concepts.equality_comparable       equality_comparable.test.cpp )
+add_cpp_test( std20.concepts.totally_ordered           totally_ordered.test.cpp )
diff --git a/src/tools/std20/concepts/test/common_reference_with.test.cpp b/src/tools/std20/concepts/test/common_reference_with.test.cpp
new file mode 100644
index 0000000..71b9b5f
--- /dev/null
+++ b/src/tools/std20/concepts/test/common_reference_with.test.cpp
@@ -0,0 +1,56 @@
+// include Catch2
+#include <catch2/catch_test_macros.hpp>
+
+// what we are testing
+#include "tools/std20/concepts.hpp"
+
+// other includes
+
+// convenience typedefs
+//using namespace njoy::tools;
+namespace std20 = nano::ranges;
+
+// test code
+struct Foo {};
+
+struct Bar : public Foo {};
+
+struct AlsoBar : private Foo {};
+
+SCENARIO( "common_reference_with" ) {
+
+  CHECK( std20::common_reference_with< Foo, Foo > );
+  CHECK( std20::common_reference_with< Bar, Foo > );
+  CHECK( std20::common_reference_with< Foo, Bar > );
+  CHECK( std20::common_reference_with< Bar, Bar > );
+  CHECK( std20::common_reference_with< int, double > );
+  CHECK( std20::common_reference_with< double, int > );
+  CHECK( std20::common_reference_with< void, void > );
+
+  CHECK( std20::common_reference_with< int&, int& > );
+  CHECK( std20::common_reference_with< const int&, int& > );
+  CHECK( std20::common_reference_with< int&, const int& > );
+  CHECK( std20::common_reference_with< int&&, int& > );
+  CHECK( std20::common_reference_with< int&&, const int& > );
+  CHECK( std20::common_reference_with< int&, int&& > );
+  CHECK( std20::common_reference_with< const int&, int&& > );
+  CHECK( std20::common_reference_with< int&&, int&& > );
+  CHECK( std20::common_reference_with< const int&&, int&& > );
+  CHECK( std20::common_reference_with< int&&, const int&& > );
+
+  CHECK( std20::common_reference_with< volatile int&, int& > );
+  CHECK( std20::common_reference_with< volatile int&, const int& > );
+  CHECK( std20::common_reference_with< const volatile int&, int& > );
+  CHECK( std20::common_reference_with< int&, volatile int& > );
+  CHECK( std20::common_reference_with< const int&, volatile int& > );
+  CHECK( std20::common_reference_with< int&, const volatile int& > );
+
+  CHECK( std20::common_reference_with< Foo&, Bar& > );
+  CHECK( std20::common_reference_with< Foo&&, Bar&& > );
+  CHECK( std20::common_reference_with< Foo&, Bar&& > );
+  CHECK( std20::common_reference_with< Foo&&, Bar& > );
+  CHECK( std20::common_reference_with< Foo&&, Bar&& > );
+
+  CHECK( ! std20::common_reference_with< AlsoBar, Foo > );   // private inheritance from Foo
+  CHECK( ! std20::common_reference_with< Foo, AlsoBar > );   // private inheritance from Foo
+} // SCENARIO
diff --git a/src/tools/std20/concepts/test/common_with.test.cpp b/src/tools/std20/concepts/test/common_with.test.cpp
new file mode 100644
index 0000000..cea7f99
--- /dev/null
+++ b/src/tools/std20/concepts/test/common_with.test.cpp
@@ -0,0 +1,56 @@
+// include Catch2
+#include <catch2/catch_test_macros.hpp>
+
+// what we are testing
+#include "tools/std20/concepts.hpp"
+
+// other includes
+
+// convenience typedefs
+//using namespace njoy::tools;
+namespace std20 = nano::ranges;
+
+// test code
+struct Foo {};
+
+struct Bar : public Foo {};
+
+struct AlsoBar : private Foo {};
+
+SCENARIO( "common_with" ) {
+
+  CHECK( std20::common_with< Foo, Foo > );
+  CHECK( std20::common_with< Bar, Foo > );
+  CHECK( std20::common_with< Foo, Bar > );
+  CHECK( std20::common_with< Bar, Bar > );
+  CHECK( std20::common_with< int, double > );
+  CHECK( std20::common_with< double, int > );
+  CHECK( std20::common_with< void, void > );
+
+  CHECK( std20::common_with< int&, int& > );
+  CHECK( std20::common_with< const int&, int& > );
+  CHECK( std20::common_with< int&, const int& > );
+  CHECK( std20::common_with< int&&, int& > );
+  CHECK( std20::common_with< int&&, const int& > );
+  CHECK( std20::common_with< int&, int&& > );
+  CHECK( std20::common_with< const int&, int&& > );
+  CHECK( std20::common_with< int&&, int&& > );
+  CHECK( std20::common_with< const int&&, int&& > );
+  CHECK( std20::common_with< int&&, const int&& > );
+
+  CHECK( std20::common_with< volatile int&, int& > );
+  CHECK( std20::common_with< volatile int&, const int& > );
+  CHECK( std20::common_with< const volatile int&, int& > );
+  CHECK( std20::common_with< int&, volatile int& > );
+  CHECK( std20::common_with< const int&, volatile int& > );
+  CHECK( std20::common_with< int&, const volatile int& > );
+
+  CHECK( std20::common_with< Foo&, Bar& > );
+  CHECK( std20::common_with< Foo&&, Bar&& > );
+  CHECK( std20::common_with< Foo&, Bar&& > );
+  CHECK( std20::common_with< Foo&&, Bar& > );
+  CHECK( std20::common_with< Foo&&, Bar&& > );
+
+  CHECK( ! std20::common_with< AlsoBar, Foo > );   // private inheritance from Foo
+  CHECK( ! std20::common_with< Foo, AlsoBar > );   // private inheritance from Foo
+} // SCENARIO
diff --git a/src/tools/std20/concepts/test/convertible_to.test.cpp b/src/tools/std20/concepts/test/convertible_to.test.cpp
index ea5bf8a..5813458 100644
--- a/src/tools/std20/concepts/test/convertible_to.test.cpp
+++ b/src/tools/std20/concepts/test/convertible_to.test.cpp
@@ -17,7 +17,7 @@ struct Bar : public Foo {};
 
 struct AlsoBar : private Foo {};
 
-SCENARIO( "derived_from" ) {
+SCENARIO( "convertible_to" ) {
 
   CHECK( std20::convertible_to< Foo, Foo > );
   CHECK( std20::convertible_to< Bar, Foo > );
@@ -26,5 +26,5 @@ SCENARIO( "derived_from" ) {
   CHECK( std20::convertible_to< void, void > );
 
   CHECK( ! std20::convertible_to< Foo, Bar > );       // Foo is not derived from Bar
-  CHECK( ! std20::convertible_to< AlsoBar, Foo > );   // private inheritance from Bar
+  CHECK( ! std20::convertible_to< AlsoBar, Foo > );   // private inheritance from Foo
 } // SCENARIO
diff --git a/src/tools/std20/concepts/test/derived_from.test.cpp b/src/tools/std20/concepts/test/derived_from.test.cpp
index 9559837..9103c11 100644
--- a/src/tools/std20/concepts/test/derived_from.test.cpp
+++ b/src/tools/std20/concepts/test/derived_from.test.cpp
@@ -26,5 +26,5 @@ SCENARIO( "derived_from" ) {
   CHECK( ! std20::derived_from< int, int > );       // same primitive types
   CHECK( ! std20::derived_from< double, double > ); // same primitive types
   CHECK( ! std20::derived_from< Foo, Bar > );       // Foo is not derived from Bar
-  CHECK( ! std20::derived_from< AlsoBar, Foo > );   // private inheritance from Bar
+  CHECK( ! std20::derived_from< AlsoBar, Foo > );   // private inheritance from Foo
 } // SCENARIO