Tic Tac Toe

Tic Tac Toe, with an opponent that actually plays properly

Three in a row on a three by three grid, except this version does the things most browser copies do not. The computer on Unbeatable genuinely cannot be beaten, there is a four by four board where you need four in a row, two people can play on the same device, and there is a neon glow mode if you would rather it did not look like a spreadsheet.

The uncomfortable fact about the standard game is that it is solved. With perfect play from both sides, every single game is a draw. That is not an opinion, it is a proven result: the whole game tree is small enough to search exhaustively, and once you know the rules of good play, there is no winning line. What makes it interesting is that most people do not know those rules, which is why the game survives on playgrounds.

How to play

1. Choose a board: 3×3 needs three in a row, 4×4 needs four.
2. Choose an opponent: the computer, or a second player on the same device.
3. Against the computer, pick a level. Unbeatable means what it says.
4. X always goes first. The running score keeps count across games.

Can you actually beat the computer?

On Unbeatable, no. It searches the complete game tree with alpha-beta pruning and plays the optimal move every time, so your best possible outcome is a draw. This has been verified against every legal sequence of human moves: 569 distinct games, zero human wins. If you draw against it, you played perfectly.

Easy will block you and take an obvious win, but plays randomly the rest of the time, so it is beatable without being pointless. Medium plays optimally most of the time and deliberately slips about one move in five, which produces the most human-feeling games of the three.

How to never lose at 3×3

• Take the centre if you go first. It sits on four of the eight winning lines, more than any other square.
• If they take the centre, take a corner. Taking an edge here is the most common losing move.
• Play corners, not edges. Corners belong to three winning lines, edges to only two.
• Look for a fork. The only way to beat an imperfect opponent is to create two threats at once, since they can only block one.
• Block before you build. Check whether they have two in a row before you look at your own line. Most losses come from missing this.

Why 4×4 is a completely different game

Four in a row on a sixteen square board is not solved the way the small version is, and it plays far more like Connect Four than like Tic Tac Toe. The board is big enough that double threats appear naturally, the centre four squares are worth far more than the corners, and games rarely end in a draw. The computer switches to a depth-limited search with a positional heuristic there, since the full tree is too large to walk, which means it is strong but genuinely beatable.

Glow mode and two player

Glow mode swaps the board to a dark surface with neon markers, which is easier on the eyes at night and considerably more satisfying to win on. Two Player passes the device back and forth with no computer involved, which is what the game was always for. Your running score is saved in your browser and no account is needed.