#5107: document that default encoding is the same as locale.getpreferredencoding().
diff --git a/Doc/library/functions.rst b/Doc/library/functions.rst
index 744a99f..b5a4fb8 100644
--- a/Doc/library/functions.rst
+++ b/Doc/library/functions.rst
@@ -666,9 +666,14 @@
Other common values are ``'w'`` for writing (truncating the file if it
already exists), and ``'a'`` for appending (which on *some* Unix systems,
means that *all* writes append to the end of the file regardless of the
- current seek position). In text mode, if *encoding* is not specified the
- encoding used is platform dependent. (For reading and writing raw bytes use
- binary mode and leave *encoding* unspecified.) The available modes are:
+ current seek position).
+
+ In text mode, if *encoding* is not specified the encoding used is the same as
+ returned by :func:`locale.getpreferredencoding`, if the :mod:`locale` module
+ is available, else ASCII. For reading and writing raw bytes, use binary mode
+ and leave *encoding* unspecified.
+
+ The available modes are:
========= ===============================================================
Character Meaning