diff --git a/src/include/units/bits/common_quantity.h b/src/include/units/bits/common_quantity.h
index 6e9c9640..30d27193 100644
--- a/src/include/units/bits/common_quantity.h
+++ b/src/include/units/bits/common_quantity.h
@@ -65,12 +65,20 @@ using common_quantity = detail::common_quantity_impl<Q1, Q2, Rep>::type;
 
 }  // namespace units
 
+#if COMP_GCC >= 10
+
 namespace std {
 
+#else
+
+namespace concepts {
+
+#endif
+
 template<units::Quantity Q1, units::Quantity Q2>
   requires units::equivalent_dim<typename Q1::dimension, typename Q2::dimension>
 struct common_type<Q1, Q2> {
   using type = units::common_quantity<Q1, Q2>;
 };
 
-}
\ No newline at end of file
+}
diff --git a/src/include/units/bits/external/hacks.h b/src/include/units/bits/external/hacks.h
index 674ca8d7..5e4f451d 100644
--- a/src/include/units/bits/external/hacks.h
+++ b/src/include/units/bits/external/hacks.h
@@ -76,6 +76,7 @@ namespace std {
   using concepts::convertible_to;
   using concepts::default_constructible;
   using concepts::derived_from;
+  using concepts::equality_comparable;
   using concepts::equality_comparable_with;
   // using concepts::floating_point;
   using concepts::integral;