diff --git a/Autotiles.md b/Autotiles.md
index 6570f42..8fbc346 100644
--- a/Autotiles.md
+++ b/Autotiles.md
@@ -19,9 +19,17 @@ sheets use:
 
 The bottom 2 × 2 tiles of either block are one fully bordered tile: its middle four quarters are
 the inside, the ring around them the borders and the convex corners. A floor block has one tile row
-more, and that row holds the four notches — the piece for a corner whose two sides are joined but
-whose diagonal is not. A wall knows no diagonals, which is why it is a row shorter and has 16 tiles
-instead of 47.
+more; its **right** tile holds the four notches — the piece for a corner whose two sides are joined
+but whose diagonal is not. The tile to the left of them is a lone tile, the same picture the four
+convex corners make, and it stays unused. A wall knows no diagonals, which is why it is a row
+shorter and has 16 tiles instead of 47.
+
+The inside is taken **turned by half a tile**: the top-left corner of a drawn tile gets the quarter
+from the bottom right of the block's inside. That is not a detail — it is what makes two tiles next
+to each other continue the drawing, because the quarters that meet on screen are the ones that touch
+in the sheet. The borders behave normally, left on the left and top on top. The rule is the one of
+RPG Maker MZ (`Tilemap.FLOOR_AUTOTILE_TABLE`), and the tests check the editor against that table
+row by row.
 
 **The tile size comes from the picture**: a block is two tiles wide by definition, so a 96 px wide
 block means 48 px tiles and 24 px quarters. There is no field for it — a second number could only