diff --git a/include/X11/extensions/XI2proto.h b/include/X11/extensions/XI2proto.h
index 4cdaa0dfb23536d10661619df7a8ae07a2aa8a50..28a9062e8e74e3f7589621a374e24b349cec3235 100644
--- a/include/X11/extensions/XI2proto.h
+++ b/include/X11/extensions/XI2proto.h
@@ -149,8 +149,9 @@ typedef struct {
 
 /**
  * Denotes button capability on a device.
- * Struct is followed by num_buttons * Atom that names the buttons in the
- * device-native setup (i.e. ignoring button mappings).
+ * Struct is followed by a button bit-mask (padded to four byte chunks) and
+ * then num_buttons * Atom that names the buttons in the device-native setup
+ * (i.e.  ignoring button mappings).
  */
 typedef struct {
     uint16_t    type;           /**< Always ButtonClass */