diff --git a/Doc/library/base64.rst b/Doc/library/base64.rst
index c9ce28b..2a9840b 100644
--- a/Doc/library/base64.rst
+++ b/Doc/library/base64.rst
@@ -16,7 +16,8 @@
 encoding algorithm is not the same as the :program:`uuencode` program.
 
 There are two interfaces provided by this module.  The modern interface supports
-encoding and decoding string objects using all three alphabets.  The legacy
+encoding and decoding string objects using all three :rfc:`3548` defined
+alphabets (normal, URL-safe, and filesystem-safe).  The legacy
 interface provides for encoding and decoding to and from file-like objects as
 well as strings, but only using the Base64 standard alphabet.