diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index 43d029a..b4e39a4 100644
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -513,6 +513,10 @@
 is slightly deceptive; on Unix platforms, sockets and pipes are also referenced
 by file descriptors.
 
+The :meth:`~file.fileno` method can be used to obtain the file descriptor
+associated with a file object when required.  Note that using the file
+descriptor directly will bypass the file object methods, ignoring aspects such
+as internal buffering of data.
 
 .. function:: close(fd)