Players have hands of four cards numbered 1 through 19. Play 1, 2, 3, or 4 cards to place a stone on the gameboard, which consists of 36 numbered squares in a non-obvious order. (The yucata implementation allows for alternate arrangements.) The goal is to claim four squares in a line, horizontally, vertically, or diagonally. But multiple stones can be placed on a square, and a square is only safe if a player has two more stones on it than any other player. If four-in-a-row is not achieved within 20 turns, the game ends in favor of the player with the most points.
Fourmation is a version of the game with a modular board published by SimplyFun.