diff --git a/src/lisp.h b/src/lisp.h
index f1ab29ef2f2..6d2fd4b83f2 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -3120,15 +3120,15 @@ struct Lisp_Fwd
   enum Lisp_Fwd_Type type : 8;
   union
   {
-    intmax_t *intvar;
-    bool *boolvar;
-    Lisp_Object *objvar;
+    intmax_t *intvar;		/* when type == Lisp_Fwd_Int */
+    bool *boolvar;		/* when type == Lisp_Fwd_Bool */
+    Lisp_Object *objvar;	/* when type == Lisp_Fwd_Obj */
     struct
     {
       uint16_t offset;
       enum Lisp_Fwd_Predicate predicate : 8;
-    } buf;
-    int kbdoffset;
+    } buf;			/* when type == Lisp_Fwd_Buffer_Obj */
+    int kbdoffset;		/* when type == Lisp_Fwd_Kboard_Obj */
   } u;
 };