diff --git a/include/pixmapstr.h b/include/pixmapstr.h
index cc5a8d8de..702faf0ac 100644
--- a/include/pixmapstr.h
+++ b/include/pixmapstr.h
@@ -74,8 +74,8 @@ typedef struct _Pixmap {
     DrawableRec		drawable;
     PrivateRec		*devPrivates;
     int			refcnt;
-    int			devKind;
-    DevUnion		devPrivate;
+    int			devKind; /* This is the pitch of the pixmap, typically width*bpp/8. */
+    DevUnion		devPrivate; /* When !NULL, devPrivate.ptr points to the raw pixel data. */
 #ifdef COMPOSITE
     short		screen_x;
     short		screen_y;