Tic-Tac-Toe

How to play

Tic-Tac-Toe is played on a three-by-three grid. You are X and go first; the computer or your opponent is O. Tap any empty square to place your mark, and players alternate turns until someone completes a line or the board fills up.

Three of your marks in a row — horizontally, vertically, or along either diagonal — wins the game immediately. If all nine squares fill without either player completing a line, the game is a draw.

Playing the computer, or a friend

Easy plays a shallow search and will miss an opponent's setup more often than not — good for a new player, or a quick game you expect to win. Medium looks further ahead and punishes an obvious mistake. Hard plays perfectly: the entire game is small enough to search completely, so a hard-difficulty opponent can never be beaten. The best a perfect opponent allows is a draw, and only if you play without a mistake yourself.

Switching the opponent to two players passes the same board back and forth on one device, with no computer involved. Your game is saved in this browser as you play, so closing the tab does not cost you the position.

Why it's always a draw between two good players

Tic-Tac-Toe is what game theorists call a "solved" game: with perfect play from both sides, every game ends in a draw. That fact was proven formally in the 1950s, but it matches the experience of anyone who has played the game seriously as a child and eventually stopped losing — there is a point past which the game stops being interesting to an adult precisely because it has no more secrets. It remains a good first game to teach, because the entire strategy can be reasoned out by a beginner in a single sitting, which is not true of tic-tac-toe's much larger cousin, Gomoku.

The two moves that matter most: take the center if it's open, and if your opponent has two in a row with the third square empty, block it immediately — that single habit is most of what separates a player who never loses from one who occasionally does.