diff --git a/src/structs.h b/src/structs.h
index 6bf2126..4aa7415 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -2273,6 +2273,8 @@
  */
 struct window_S
 {
+    int		w_id;		    /* unique window ID */
+
     buf_T	*w_buffer;	    /* buffer we are a window into (used
 				       often, keep it the first item!) */