diff --git a/src/frame.h b/src/frame.h
index fa22b2c875d..5cab1f42c5d 100644
--- a/src/frame.h
+++ b/src/frame.h
@@ -482,7 +482,8 @@ struct frame
 
   /* frame opacity
      alpha[0]: alpha transparency of the active frame
-     alpha[1]: alpha transparency of inactive frames   */
+     alpha[1]: alpha transparency of inactive frames
+     Negative values mean not to change alpha.  */
   double alpha[2];
 
   /* Exponent for gamma correction of colors.  1/(VIEWING_GAMMA *