From the introduction: The Tunnel Double Passage group of Naked Geomorphs contains 18 tiles, each with two pairs of connected tunnel entrances (T), and rock (R) at the other two positions. Sets A and B are completely interchangeable. I.e., tile T119A can be replaced with tile T119B, without changing the basic structure of your map. Set A has narrower tunnels than set B, with lots of one-hex-wide passages and occasional small galleries.