diff --git a/docs/ipc b/docs/ipc
index 83ab7218..54e7fa1a 100644
--- a/docs/ipc
+++ b/docs/ipc
@@ -1,7 +1,7 @@
 IPC interface (interprocess communication)
 ==========================================
 Michael Stapelberg <michael@i3wm.org>
-February 2012
+July 2012
 
 This document describes how to interface with i3 from a separate process. This
 is useful for example to remote-control i3 (to write test cases for example) or
@@ -296,6 +296,11 @@ window_rect (map)::
 geometry (map)::
 	The original geometry the window specified when i3 mapped it. Used when
 	switching a window to floating mode, for example.
+window (integer)::
+	The X11 window ID of the *actual client window* inside this container.
+	This field is set to null for split containers or otherwise empty
+	containers. This ID corresponds to what xwininfo(1) and other
+	X11-related tools display (usually in hex).
 urgent (bool)::
 	Whether this container (window or workspace) has the urgency hint set.
 focused (bool)::