diff --git a/Doc/c-api/init.rst b/Doc/c-api/init.rst
index 561c8a1b879bae..2de596f282c49a 100644
--- a/Doc/c-api/init.rst
+++ b/Doc/c-api/init.rst
@@ -2226,6 +2226,13 @@ is resumed, and its locks reacquired.  This means the critical section API
 provides weaker guarantees than traditional locks -- they are useful because
 their behavior is similar to the :term:`GIL`.
 
+Critical sections are appropriate for protecting custom types defined by your
+C-API extensions. They should generally not be used with built-in types like
+:c:struct:`PyDictObject` or :c:struct:`PyListObject` because their public APIs
+already use critical sections internally, with the :ref:`notable
+exception<PyDict_Next>` of :c:func:`PyDict_Next`, which requires external
+locking.
+
 The functions and structs used by the macros are exposed for cases
 where C macros are not available. They should only be used as in the
 given macro expansions. Note that the sizes and contents of the structures may