diff --git a/Doc/library/codecs.rst b/Doc/library/codecs.rst
index 1c10462c1509c3..7d56327aad265e 100644
--- a/Doc/library/codecs.rst
+++ b/Doc/library/codecs.rst
@@ -189,7 +189,8 @@ wider range of codecs when working with binary files:
 
    .. note::
 
-      Underlying encoded files are always opened in binary mode.
+      If *encoding* is not ``None``, then the
+      underlying encoded files are always opened in binary mode.
       No automatic conversion of ``'\n'`` is done on reading and writing.
       The *mode* argument may be any binary mode acceptable to the built-in
       :func:`open` function; the ``'b'`` is automatically added.
diff --git a/Lib/codecs.py b/Lib/codecs.py
index e6ad6e3a052364..3b173b612101e7 100644
--- a/Lib/codecs.py
+++ b/Lib/codecs.py
@@ -878,7 +878,8 @@ def open(filename, mode='r', encoding=None, errors='strict', buffering=-1):
         codecs. Output is also codec dependent and will usually be
         Unicode as well.
 
-        Underlying encoded files are always opened in binary mode.
+        If encoding is not None, then the
+        underlying encoded files are always opened in binary mode.
         The default file mode is 'r', meaning to open the file in read mode.
 
         encoding specifies the encoding which is to be used for the
diff --git a/Lib/test/test_codecs.py b/Lib/test/test_codecs.py
index a9c43d9554beaf..8edd5ac0633e36 100644
--- a/Lib/test/test_codecs.py
+++ b/Lib/test/test_codecs.py
@@ -705,7 +705,8 @@ def test_decoder_state(self):
                                          "spamspam", self.spambe)
 
     def test_bug691291(self):
-        # Files are always opened in binary mode, even if no binary mode was
+        # If encoding is not None, then
+        # files are always opened in binary mode, even if no binary mode was
         # specified.  This means that no automatic conversion of '\n' is done
         # on reading and writing.
         s1 = 'Hello\r\nworld\r\n'