Ultimate Tic Tac Toe is a strategic game for two players. The game commences with Player X, who always takes the first turn. The key rule is that the next player must place their marker in the larger square that corresponds to the smaller square of the previous player’s move. However, if the larger square has already been won by either X or O, and the previous move was made in that square, the next player is allowed to place their marker in any available square on the board. This unique rule introduces an additional layer of strategy to the game.