diff --git a/Doc/library/tarfile.rst b/Doc/library/tarfile.rst
index f25af8c..c34f2c4 100644
--- a/Doc/library/tarfile.rst
+++ b/Doc/library/tarfile.rst
@@ -290,9 +290,10 @@
 
       *fileobj* is not closed, when :class:`TarFile` is closed.
 
-   *format* controls the archive format. It must be one of the constants
+   *format* controls the archive format for writing. It must be one of the constants
    :const:`USTAR_FORMAT`, :const:`GNU_FORMAT` or :const:`PAX_FORMAT` that are
-   defined at module level.
+   defined at module level. When reading, format will be automatically detected, even
+   if different formats are present in a single archive.
 
    The *tarinfo* argument can be used to replace the default :class:`TarInfo` class
    with a different one.