Publisher's blurb: 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 T119B can be replaced with tile T119A, without changing the basic structure of your map. Set B has broader tunnels than set A, with no one-hex-wide constrictions on the main passages.