What is Ultimate Tic-Tac-Toe?
Ultimate Tic-Tac-Toe is played on a big 3×3 board whose nine squares each hold a small tic-tac-toe board. X moves first and may play anywhere. After that, the square you mark inside a small board sends your opponent to the matching small board of the big one: play in the top-right square of any small board, and your opponent must play next in the top-right small board.
A small board is won with three in a row inside it, and it then counts as that player's square on the big board. A small board that fills up without a line counts for nobody. Once a small board is won or full, no more moves go there, and a player sent to it may play in any open small board instead. You win the game by winning three small boards in a row on the big board. If every small board is decided without such a line, the game is a draw.
The sending rule is what makes the game. A good move on one small board is a bad one if it sends your opponent somewhere they want to be, and sending them to a board that is already decided hands them a free choice of where to play.
Rule changes
| Rule | Standard Tic-Tac-Toe | Ultimate Tic-Tac-Toe |
|---|---|---|
| Board | Standard Tic-Tac-ToeOne 3×3 board, or 4×4 and 5×5 in the table setup. | Ultimate Tic-Tac-ToeA big 3×3 board of nine small 3×3 boards: 81 squares in all. |
| Where you play | Standard Tic-Tac-ToeAny empty square. | Ultimate Tic-Tac-ToeIn the small board that matches the square of your opponent's last move; in any open small board on the first move, or when that board is won or full. |
| Small boards | Standard Tic-Tac-ToeThere is only one board. | Ultimate Tic-Tac-ToeThree in a row wins a small board. A full small board without a line counts for nobody, and a decided board takes no more moves. |
| How to win | Standard Tic-Tac-ToeComplete a line of three of your marks. | Ultimate Tic-Tac-ToeWin three small boards in a line on the big board. |
| Draws | Standard Tic-Tac-ToeA full board without a line is a draw. | Ultimate Tic-Tac-ToeWhen every small board is decided and nobody has three in a line on the big board, the game is a draw. |
History
Ultimate Tic-Tac-Toe is also known as super tic-tac-toe, meta tic-tac-toe, strategic tic-tac-toe and (tic-tac-toe)². Its origins are unknown. It became widely known after the mathematics teacher Ben Orlin described it on his blog in June 2013, and it has since been the subject of papers in mathematics and computer science.
Unlike ordinary tic-tac-toe, the game is far too large to solve by checking every position. Researchers showed in 2020 that the first player has a winning strategy in a version with a different rule, in which players must keep playing in small boards that are already decided. That is not the rule played here, where a decided board is closed.