diff --git a/av1/common/scale.h b/av1/common/scale.h
index fd30416..28bcec4 100644
--- a/av1/common/scale.h
+++ b/av1/common/scale.h
@@ -52,6 +52,7 @@
          (sf->x_scale_fp != REF_NO_SCALE || sf->y_scale_fp != REF_NO_SCALE);
 }
 
+// See AV1 spec, Section 6.8.6. Frame size with refs semantics.
 static INLINE int valid_ref_frame_size(int ref_width, int ref_height,
                                        int this_width, int this_height) {
   return 2 * this_width >= ref_width && 2 * this_height >= ref_height &&