diff --git a/vp9/common/vp9_blockd.h b/vp9/common/vp9_blockd.h
index 951e6e023..7f8c5f215 100644
--- a/vp9/common/vp9_blockd.h
+++ b/vp9/common/vp9_blockd.h
@@ -98,6 +98,9 @@ typedef struct {
   int_mv as_mv[2];  // first, second inter predictor motion vectors
 } b_mode_info;
 
+// Note that the rate-distortion optimization loop, bit-stream writer, and
+// decoder implementation modules critically rely on the enum entry values
+// specified herein. They should be refactored concurrently.
 typedef enum {
   NONE = -1,
   INTRA_FRAME = 0,