diff --git a/src/tetris.c b/src/tetris.c
index f6ab3ae..c842eb4 100644
--- a/src/tetris.c
+++ b/src/tetris.c
@@ -75,7 +75,7 @@ void tetris_place(u8 *restrict *restrict rows, u8 tetromino, uint x, uint y) {
 	uint *end = pos + 8;
 	tetris_get_blocks(tetromino, pos);
 	tetromino &= 7;    /* strip rotation data */
-	tetromino |= 0x80; /* set an undefined bit, to signify there is data here. */
+	tetromino |= TET_NON_EMPTY;
 	for (; pos < end; pos += 2)
 		rows[*(pos + 1)][*pos] = tetromino;
 }
diff --git a/src/tetris.h b/src/tetris.h
index cb299af..28c4ae4 100644
--- a/src/tetris.h
+++ b/src/tetris.h
@@ -6,9 +6,11 @@
 #define TET_WIDTH  10
 #define TET_HEIGHT 24 /* height may be 16—24 */
 
-/* Defines tetromino id.
+/* Defines tetromino indices.
  * The `TET_R*` definitions specify various rotations.
- * This is designed to be OR'd with the tetromino shape index. */
+ * This is designed to be OR'd with the tetromino shape index.
+ * Shape index can be obtained via `val & 7`, and rotation index is
+ * obtained via `val >> 3`. */
 enum tetromino {
 	TET_I = 0x00,
 	TET_O = 0x01,
@@ -21,6 +23,11 @@ enum tetromino {
 	TET_R90 = 0x08,
 	TET_R180 = 0x10,
 	TET_R270 = 0x18,
+
+	/* Used when storing blocks to the board;
+	 * allows to identify non-empty indices from empty ones.
+	 * This value should not be used during general computation. */
+	TET_NON_EMPTY = 0x80,
 };
 
 /* Stores the co-ordinates of the four blocks in a 4x4 plane.