Fifteen is played with 9 stones numbered 1 to 9, on a board consisting of 3x3 squares. The goal of Fifteen is to place 15 in a full line (horizontally, vertically, diagonally).
Taking turns, the players pick one of the available stones and place it on an empty square.
The game ends when the sum of 3 adjacent stones (horizontally, vertically or diagonally) is exactly 15. The first player to do this wins the game.