A kind of vertical tic-tac-toe, with a plastic rack into which checkers can be slipped. Each player wants to get four checkers of their color in a row (horizontal, vertical, or diagonal) and block the other player from getting four in a row. Manufacturer Milton Bradley recommends it for ages seven to fourteen, but I think people much older or younger can enjoy it (unless they're the type who'll dump the checkers out of the rack rather than lose). There's now also an electronic hand-held version.