Tic-Tac-Toe

How to play tic-tac-toe

Tic-tac-toe (also called noughts and crosses) is a two-player game on a 3 by 3 grid. One player is X and the other is O, and X always goes first.

• Take turns putting your mark in any empty square.
• Get three of your marks in a row to win: across, down, or corner to corner.
• If all nine squares fill up with no three in a row, the game is a draw.

Playing on Poozee

• vs Computer: pick 1st to play X or 2nd to play O, and choose a level. Easy often misses a win or forgets to block. Medium always takes a win and always blocks, but never looks further ahead, so it can be caught by a fork. Hard has searched every possible game and never loses: the best you can do against it is a draw.
• 2 Players: share one screen and take turns. X starts every game.
• Hint shows the best move for whoever's turn it is, with the reason: a win, a block, a fork, or the safe move that gives your opponent the best chance to go wrong.
• Undo takes back your last move (against the computer, along with its reply).
• The scoreboard keeps your wins, losses and draws for each level, plus your unbeaten run: games in a row without a loss.

On a keyboard, the number keys work like a numpad: 7 8 9 are the top row, 4 5 6 the middle and 1 2 3 the bottom.

Strategy basics

1. Win if you can, block if you must. Check both before every move: a missed block is the easiest way to lose.
2. Make a fork. A fork is a move that creates two ways to win at once. Your opponent can block only one, so you win with the other. Every win against a player who always blocks comes from a fork.
3. Going first, start in a corner. With perfect play every opening is a draw, but a corner leaves O only one safe reply (the center). Every other answer loses.
4. Going second, answer a corner with the center and the center with a corner. Then watch for the opposite-corner trap: if X takes the corner across from its first one, play an edge, not a corner.

Worked example: the opposite-corner trap

X starts in the top left corner and O takes the center. X now plays the bottom right corner, across from the first. It looks harmless, and O's natural reply is another corner. That loses: X must block O's diagonal, and the block itself makes two lines of two for X at once. O can stop only one. We checked every reply with Poozee's game engine: after X's second corner, both remaining corners lose for O, and all four edges hold the draw.

For more traps, the best first move and how O should answer each opening, read how to win tic-tac-toe.

Noughts and crosses, and the first computer games

Tic-tac-toe is the American name; in Britain the same game is called noughts and crosses. It was also one of the first games ever played against a computer: in 1952, A. S. Douglas wrote a noughts and crosses program called OXO for the EDSAC computer at the University of Cambridge.

The game is small enough for a computer to look at everything. Only 5,478 different positions can come up in a game, and Poozee's Hard level has worked out the result of every one of them, which is why it can't be beaten.