Tic-Tac-Toe

How to play

1. Choose a Mode: Against the computer or Two players (you pass the device back and forth).
2. Against the computer, pick a Difficulty (Easy, Medium or Hard (unbeatable)) and choose which side You play: X (you start) or O (computer starts).
3. Click or tap an empty square to place your mark. X always moves first, then the players alternate.
4. The first player to line up three marks in a row, a column or a diagonal wins, and the winning line lights up. If all nine squares fill up without a line, the game is a draw.
5. Press New game (or the N key) for another round. Changing the mode, difficulty or side also starts a new game.

The counters under the board keep a running tally of X wins, O wins and Draws in this browser. Best unbeaten run is your longest series of games without a loss against the computer at the selected difficulty. Reset score sets everything back to zero.

Playing with the keyboard

The board is a 3 × 3 group of buttons. Press Tab to enter it, then move with the arrow keys and press Enter or Space to play the highlighted square. You can also press the digits 1 to 9: they are laid out like a phone keypad, so 1 is the top-left square, 5 the centre and 9 the bottom-right square. Every square announces its row, column and content to screen readers.

How the computer plays

• Easy picks any free square at random. It is a relaxed opponent for children or for learning the rules.
• Medium always completes its own line when it can and blocks yours when you threaten to win. Otherwise it plays the best move two times out of three and a random move the rest of the time, so it can be beaten with a well-prepared fork.
• Hard (unbeatable) uses the minimax algorithm: it looks at every possible continuation of the game until the end and picks a move with the best guaranteed result, preferring faster wins. Tic-tac-toe is a solved game: with perfect play on both sides it always ends in a draw. That means the best you can do against Hard is draw every game, which is exactly what the unbeaten run rewards.

Strategy tips

• Start in a corner. As X, a corner opening gives your opponent the most ways to go wrong. The only safe reply for O is the centre.
• Take the centre if you can. The centre square belongs to four possible lines (a row, a column and both diagonals), more than any other square.
• Create a fork. A fork is a move that threatens two lines at once. Your opponent can only block one, so you win on the next turn. Forks are the only way to beat a player who always blocks.
• Block forks in advance. As O against a corner opening, after taking the centre, avoid playing a corner that lets X set up two threats; an edge move that forces X to respond is often safer.
• Always check for an immediate win first, then for an immediate threat to block, before thinking about anything else.