Tic-Tac-Toe Strategy: How to Never Lose

By Nesim Razon, independent Android developer

Tic-Tac-Toe is a solved game: with correct play by both sides it always ends in a draw. That means you can learn a few rules and never lose. Here are the rules and the strategy, which you can practice against the computer on Hard.

The rules

Two players take turns placing X and O on a 3x3 grid. The first player to get three of their marks in a row, horizontally, vertically or diagonally, wins. If the grid fills with no winner, it is a draw.

If you go first

• Start in a corner or in the center. A corner gives the opponent the most chances to go wrong.
• If the opponent does not answer in the center, you can often force a win.
• After your first move, aim to create a fork: a move that creates two threats of three in a row at once. The opponent can block only one.

If you go second

• If the opponent starts in the center, take a corner.
• If the opponent starts in a corner, take the center.
• Never answer a corner or a center opening with an edge: against correct play, that loses.

Always, on every move

• First check whether you can win this move. If you can, do it.
• Then check whether the opponent threatens to win and block it.
• Then look for a fork for yourself, and block the opponent's forks.
• If nothing else applies, take the center, then a corner, then an edge.