diff --git a/src/cu.h b/src/cu.h
index 4b68bca4..2269fb1f 100644
--- a/src/cu.h
+++ b/src/cu.h
@@ -153,6 +153,24 @@ typedef struct {
    * - Left reference CUs on column 0.
    * - All of LCUs CUs on 1:9, 1:9.
    * - Top right reference CU on the last slot.
+   *
+   \verbatim
+
+      .-- left reference CUs
+      v
+       0 |  1  2  3  4  5  6  7  8 | 81 <-- top reference CUs
+     ----+-------------------------+----
+       9 | 10 11 12 13 14 15 16 17 |
+      18 | 19 20 21 22 23 24 25 26 <-- this LCU
+      27 | 28 29 30 31 32 33 34 35 |
+      36 | 37 38 39 40 41 42 43 44 |
+      45 | 46 47 48 49 50 51 52 53 |
+      54 | 55 56 57 58 59 60 61 62 |
+      63 | 64 65 66 67 68 69 70 71 |
+      72 | 73 74 75 76 77 78 79 80 |
+     ----+-------------------------+----
+
+   \endverbatim
    */
   cu_info_t cu[9*9+1];
 } lcu_t;