How to Play Tic Tac Toe

1. 1Draw the grid and pick marksTwo lines down and two across make nine squares. One player is X, the other is O. X always goes first, which is the only advantage either player gets.
2. 2Take turns, one mark eachWrite your mark in any empty square. You may not pass, you may not move a mark you have already written, and you may not use a square your opponent has taken.
3. 3Watch both sets of marksBefore each turn, look for your own two-in-a-row with an empty third square, then for your opponent's. Take the win if you have one; block theirs if you do not.
4. 4Stop at the lineThe moment three of one mark sit in a straight line, that player has won and the game is over, even with empty squares left.
5. 5Call a full grid a drawNine marks and no line is a drawn game. Nobody scores. Between two players who know what they are doing, this is how nearly every game ends.

There is no way to force a win against a correct opponent — the game is a proven draw. What there is, is a way to never lose, and a way to punish every mistake. Both come down to a fixed order of questions to ask on each turn.

• Can I win now? If two of your marks share a line with one empty square, take it.
• Can they win now? If the same is true for your opponent, block it.
• Can I fork? Look for a square that would give you two winning squares at once. That is the only mark that beats an alert opponent.
• Which of the rest is safe? Of the squares left, rule out every one that would leave your opponent a fork on their next turn — then, among what survives, take the centre, then a corner, then an edge, in that order of how many lines they sit on: four, three, two.

Those four questions are a complete defence: a player who follows them never loses, from either side of the board, against anything at all. The test suite behind this page proves it the only way it can be proved — by playing that exact list against every reply that exists, both as X and as O.

The last question is the one that is usually written down wrong, and the order inside it is doing real work. "Centre, then corner, then edge" on its own loses: after X takes a corner, you take the centre and X takes the opposite corner, every free corner hands X a fork, and only an edge saves the game. Checking for their fork first, and only then falling back to the centre-corner-edge order, is what turns a rule of thumb into a strategy. One shortcut: if your move makes a threat they are forced to answer, the only fork that can still hurt you is one on the very square they have to play.

1. 1X takes a cornerSay the top-left square. Seven of O's eight possible replies now lose with correct play from X; only the centre saves the game.
2. 2O replies on an edgeSay the top-middle square. Every reply except the centre leaves X with a forced win, and an edge is the cleanest of the seven to punish — the two corner replies need an adjustment, which the last step comes back to.
3. 3X takes the centreX now holds two of the three squares on the long diagonal and the third is empty — an immediate threat that O has no choice but to block. This is the step that fails against the opposite corner: O would already be sitting on that diagonal, so there would be nothing to threaten.
4. 4O blocks the diagonalForced. O has no choice, which is the point: the fork is built while your opponent is busy answering a threat they cannot ignore.
5. 5X plays the forking squareAfter an edge reply that square is a free corner: it completes two different lines-in-waiting at once, O can block only one, and X plays the other next turn. Against a corner reply the same position arrives but the forking square is an edge instead — the shape is identical, the square is not.

The defence against the corner opening is simply to take the centre, and then to answer each threat with the square that does not build a second line for X. Play tic tac toe against the Sharp level to see the fork work — it blocks every single threat and still loses, because it never looks one move past the mark you just played.

Four in a row wins on both, and the shape of the game changes with the number of lines. The 4 × 4 board has only ten — four rows, four columns and the two full diagonals — so almost every square is contested and the board fills quickly. The 5 × 5 board has 28, because a run of four can start anywhere along a line rather than having to fill it, and that extra room makes double threats routine rather than rare.

One rule catches people out on the bigger boards: three in a row is worth nothing there. Only four counts, so a line of three that cannot be extended to four — because the ends are blocked or the board runs out — is dead, exactly like a line with one of each mark in it. If you liked the bigger boards, Four in a Row is the same idea with gravity, and Reversi is the other classic where one move can change the whole board.