Ultimate Tic-tac-toe
Tic-tac-toe with one rule added, and the rule turns a solved four-second game into one you can think about for an hour. You are not choosing a square. You are choosing where they have to play.
The board
- Nine tic-tac-toe boards sit in a three by three grid, eighty-one squares in all.
- One player is X and the other O. The seed decides who is X and goes first.
- Squares are named by column and row, a1 to i9, so e5 is the middle square of the middle board.
The send, which is the whole game
- Your first move may go anywhere.
- After that, the position of your square WITHIN its small board decides which board the other player must use next.
- Play the top left square of any board and they must play in the top left board. Play the middle square and they must play in the middle board.
- So every move you make is two decisions: where to play, and where to send them.
When you may play anywhere
- If the board you are sent to has already been decided, you may play in any board that is still open.
- A board is decided when somebody has won it, or when it has filled up with no line.
- A decided board is closed for the rest of the game: no further squares in it can be played, and it cannot change hands.
- Sending the other player to a decided board hands them a free choice, which is the largest thing you can give away in this game.
Winning a board, and winning the game
- Three of your marks in a line on a small board wins that board, exactly as in ordinary tic-tac-toe.
- The board is then marked as yours on the big grid.
- Three boards of your own in a line on the big grid wins the game.
- A small board that fills up with no line belongs to nobody, and it counts for neither player.
How a game ends
- Three boards in a row wins immediately.
- If no board is left to play and nobody has three in a row, the game is a draw, and a draw is recorded as a draw rather than as a loss for anybody.
- A resignation or a missed deadline ends the game against the player who resigned or ran out of time.
Is it solved, like tic-tac-toe?
- Yes, and it does not matter, which is the interesting part.
- In 2020 a group of researchers showed that the first player can force a win with perfect play, in at most forty-three moves.
- That proof took a computer. Nobody can play it over the board, and the bot here cannot either.
- Measured on this site, the player who moves first wins about as often as the player who moves second, so in practice the game is decided by who plays better.
- Ordinary tic-tac-toe is solved in a way you can feel in ten minutes. This one is solved in a way nobody will ever notice.
The seed decides who plays X and therefore who moves first, and the seed is published when the game ends, so it can be checked afterwards.
Ultimate Tic-tac-toe terms
- The big grid
- The three by three grid of the nine boards. Winning three of them in a line wins the game.
- A decided board
- A board that has been won, or that has filled up with no line. It is closed, and being sent to one lets you play anywhere.
- A free choice
- A turn on which you may play in any open board, because the board you were sent to is decided.