diff --git a/libcxx/docs/UsingLibcxx.rst b/libcxx/docs/UsingLibcxx.rst
--- a/libcxx/docs/UsingLibcxx.rst
+++ b/libcxx/docs/UsingLibcxx.rst
@@ -429,3 +429,21 @@
 * ``identity::operator()``
 * ``to_integer``
 * ``to_underlying``
+
+
+Extended integral type support
+------------------------------
+
+Several platforms support the 128-bit integral types ``__int128_t`` and
+``__uint128_t``. When these types are present they can be used in the headers
+as required by the Standard:
+
+* ``<bits>``
+* ``<charconv>``
+* ``<functional>``
+* ``<type_traits>``
+
+As an extension these types can be used in the following headers:
+
+* ``<format>``
+* ``<random>``