The Tunnel Passage group of Naked Geomorphs contains 12 tiles, each with a pair of connected tunnel entrances (T), and rock (R) at the remaining four positions. There are actually nine different interconnection patterns; this group contains two tiles for each pattern where the tunnel positions are adjacent, because those are so useful as edge and corner tiles. Sets A and B are completely interchangeable. I.e., tile T77A can be replaced with tile T77B, without changing the basic structure of your map. Set A has narrower tunnels than set B, with lots of one-hex-wide passages and small to medium galleries.