diff --git a/Doc/library/select.rst b/Doc/library/select.rst
index 6dbf16f..bffb9cb 100644
--- a/Doc/library/select.rst
+++ b/Doc/library/select.rst
@@ -94,6 +94,15 @@
       library, and does not handle file descriptors that don't originate from
       WinSock.
 
+.. attribute:: select.PIPE_BUF
+
+   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.
+
+   .. versionadded:: 2.7
+
 
 .. _epoll-objects: