diff --git a/vp9/encoder/vp9_avg.c b/vp9/encoder/vp9_avg.c
index 223c923..99bc12e 100644
--- a/vp9/encoder/vp9_avg.c
+++ b/vp9/encoder/vp9_avg.c
@@ -142,9 +142,11 @@
   }
 }
 
+// width: value range {16, 32, 64}.
 int16_t vp9_int_pro_col_c(uint8_t const *ref, const int width) {
   int idx;
   int16_t sum = 0;
+  // sum: 14 bit, dynamic range [0, 16320]
   for (idx = 0; idx < width; ++idx)
     sum += ref[idx];
   return sum;