diff --git a/doc/protocol.txt b/doc/protocol.txt
index c051fb3..dee74ae 100644
--- a/doc/protocol.txt
+++ b/doc/protocol.txt
@@ -29,3 +29,12 @@ Packet structure, uses network byte order (big endian):
                         0 button is released ("up")
                         1 button is pressed ("down")
 
+XInput will ignore BTN_TOUCH events if they are not preceeded by
+a BTN_TOOL_PEN event -- this would never happen for a real stylus,
+because it would imply the stylus is touching the pad, yet too far
+away from the pad to be detected.
+
+A GfxTablet client must therefore be careful to send a "Button -1
+down" event before a "Button 0 down" event, to emulate this
+behaviour. If they are faking these events, they'll probably want
+to likewise send a "Button -1 up" event after a "Button 0 up" event.