Dominoes

This refers to the double–six set of dominoes (the commonest form).

According to Wikipedia, "The most common domino sets commercially available are double six (with 28 tiles) and double nine (with 55 tiles). Larger sets exist and are popular for games involving several players or for players looking for long domino games."

Wikipedia adds that the number of dominoes in a double–n set is (n + 1) times (n + 2), divided by 2. For a double–six set, 7 times 8 divided by 2 is 28; for a double–nine set, 10 times 11 divided by 2 is 55.