diff --git a/Doc/library/codecs.rst b/Doc/library/codecs.rst
--- a/Doc/library/codecs.rst
+++ b/Doc/library/codecs.rst
@@ -969,9 +969,10 @@
    Some common encodings can bypass the codecs lookup machinery to
    improve performance.  These optimization opportunities are only
    recognized by CPython for a limited set of aliases: utf-8, utf8,
-   latin-1, latin1, iso-8859-1, mbcs (Windows only), ascii, utf-16,
-   and utf-32.  Using alternative spellings for these encodings may
-   result in slower execution.
+   latin-1, latin1, iso-8859-1, mbcs (Windows only), ascii, us-ascii,
+   utf-16, and utf-32, and the same using underscores instead of dashes.
+   Using alternative spellings for these encodings may result in slower
+   execution.
 
 Many of the character sets support the same languages. They vary in individual
 characters (e.g. whether the EURO SIGN is supported or not), and in the