I learned about this super-fun variant of tic-tac-toe, the Ultimate tic-tac-toe, from a game event at Continuum ‘25, the annual fest of Maths Club at IISERB.

The game consists of nine tic-tac-toe boards arranged in a 3x3 grid. The rules are as follows:

1. The game starts with an empty board. The first player can place their mark (X or O) in any square on any of the smaller boards.
2. Your move dictates where your opponent plays next. The square you choose in the small board determines which large board square your opponent must play in.
3. To win a small board, get three of your marks in a row (horizontally, vertically, or diagonally).
4. Winning a small board claims that square on the large board.
5. To win the game, claim three squares in a row on the large board.
6. If sent to a full or won board, the player can choose any available square on the entire board.
7. If a small board ends in a tie, either player cannot claim it. (There is a crazy variant where both players can claim it).
8. The game ends when a player wins the large board or when all squares are filled, resulting in a tie.

Give the game a try here: play against a computer or play multiplayer with a friend!

If you don’t know, the classic tic-tac-toe is a solved game; that is, the outcome of a game can be correctly predicted from any position, assuming that players use an optimal strategy. With perfect play, tic-tac-toe is always a draw.

This is not the case with Ultimate tic-tac-toe; this is what intrigued me the most about the game. The first time you play it, you realize how hard it is to have a winning strategy. With each move, you play in one of the nine boards, which means you are playing multiple games simultaneously. It is challenging to keep track of the moves and calculate possible future variations. You wouldn’t be able to evaluate a position for the first few initial moves. The game keeps becoming complex, move after move; after the game, you would probably wonder how the heck you ended at the final position.

In the event, I won the first round and lost in the second 🥲.

It seems that this game would make an interesting project, like analyzing winning strategies, evaluating and solving positions, etc. There must be some interesting mathematics behind it. I’m sure many would have tried exploring the game, will have to check and see what they’ve found.

So, now onwards, if you ever see someone play your normal tic-tac-toe, teach them this variant and bring some chaos in their lives!

Wanna read more about the game? Check out Wikipedia and have a look at other variants of tic-tac-toe here.