Ultimate Tic-Tac-Toe

Common questions

How do you play ultimate tic-tac-toe?

The big board is a 3 by 3 grid of small tic-tac-toe boards. Players take turns placing X or O in one cell of a small board, and the position of that cell inside its small board picks the small board the opponent must play in next: take the top-right cell of any board and your opponent must play in the top-right board. The first move can go anywhere. Win a small board with three in a row as usual, and win the game with three small boards in a row. Here boards and cells are both numbered 1 to 9 from the top left, so a move written 5.1 is the top-left cell of the centre board.

What happens if you are sent to a board that is already won or full?

You may play in any small board that is still unfinished. The tool shows this in the side pane, for example O to board 5, already won by X, so O may play in any open board, and every open board lights up. A small board that has been won or filled takes no more moves, so a try in one is refused with the reason.

Can you keep playing in a small board after it has been won?

Not here. Some groups play on in won boards, but this tool follows the common rule that a won board is closed: its empty cells stay empty, it shows the winner's mark across it, and being sent there lets you choose any unfinished board instead.

How do you win ultimate tic-tac-toe, and can it end in a draw?

Win three small boards in a line, across, down or diagonally, and the game ends at once. A small board that fills up with no three in a row counts for nobody and blocks any line through it. If every small board is won or full and neither side has three in a line, the game is a draw. There is no tiebreak by counting boards.

Can I play ultimate tic-tac-toe against the computer?

Yes, choose Computer. The side that starts plays X, so you choose You start or Computer starts. The computer is a beginner: it searches at most three moves ahead (its move, your reply and its next move) for up to 750 ms in a background worker, scores positions by small boards won, two-in-a-rows and won boards lined up on the big board, and picks at random among moves it scores almost the same. It takes a win when it sees one, but it is not perfect play. If the background search cannot run in your browser, the computer plays a quick rule-based legal move instead and says so.

Can two people play on one device?

Yes, choose Two players and take turns on the same screen, with X or O starting as you pick. There is no online play and no account. Changing the mode or who starts in the middle of a game asks first, because it begins a new game.

Can I undo a move or save a game?

Undo takes back the last move with two players. Against the computer it takes back your move and the computer's reply, so it is your turn again. The game in progress is kept in this browser and comes back when you return, and Save position keeps up to six positions, each with its mode, whose turn it is and the board they must play in. Nothing is sent anywhere, and Start over at the top of the page forgets the game and the saved positions.

Can I play with a keyboard?

Yes. Tab to the board, then use the arrow keys to move between all 81 cells, Home and End to jump along a row, and Enter or Space to play. A cell you cannot play is still reachable, and pressing it says why the move is refused.

What strategy helps in ultimate tic-tac-toe?

Before choosing a cell, check the board it sends your opponent to. Taking a local win may still give them a useful destination. Look for two-in-a-row threats both inside small boards and across the big board, and protect the small boards that complete your larger line. Sending a player to a won or full board gives them free choice among unfinished boards. These are planning ideas, not a guaranteed winning strategy.

Which algorithm and evaluation does the computer use?

It uses negamax, a form of minimax that scores each position from the side to move, with alpha-beta pruning to skip branches that cannot improve the current choice. Its evaluation also gives weight to centre cells and to having a free choice of destination board. These help it compare legal moves within the practical search limit described above.