diff --git a/Doc/library/select.rst b/Doc/library/select.rst
index bffb9cb..eea442b 100644
--- a/Doc/library/select.rst
+++ b/Doc/library/select.rst
@@ -99,7 +99,7 @@
    Files reported as ready for writing by :func:`select`, :func:`poll` or
    similar interfaces in this module are guaranteed to not block on a write
    of up to :const:`PIPE_BUF` bytes.
-   This value is guaranteed by POSIX to be at least 512.
+   This value is guaranteed by POSIX to be at least 512.  Availability: Unix.
 
    .. versionadded:: 2.7