How to win at tic-tac-toe

On the classic board, winning means waiting for a mistake and knowing it when you see one. On the big board it means building threats faster than your opponent can block them. This guide covers both: the openings and traps of 3×3, how to hold 4×4, and how five in a row is really won.

Why 3×3 is always a draw

There are only 255,168 different games of tic-tac-toe, and computers have played through every one. With the best play on both sides, every game is a draw. Crosses cannot force a win, and noughts cannot either.

The reason is simple. A single threat — two in a row with the third square free — is always blocked. The only way to win is a fork: two threats at once, so that the opponent can block only one of them. On a board of nine squares a careful defender can always see a fork coming and stop it a move earlier. So the game is decided by mistakes, and your job is to make your opponent's mistakes likely and your own impossible.

Open in a corner

Crosses have three kinds of first move: the centre, a corner or an edge. None of them wins against good play, but they are not equally dangerous:

after a corner, seven of noughts' eight answers lose — only the centre holds;
after the centre, four of the eight lose (all the edges);
after an edge, four of the eight lose.

So a corner gives your opponent the most ways to go wrong, and many people answer it with an edge or the opposite corner without thinking. Open in a corner.

Answering as noughts

As noughts you are defending from the first move, and two rules cover almost everything:

Against a corner, take the centre. It is the only answer that does not lose.
Against the centre, take a corner. Any corner will do; an edge loses.

Crosses open in a corner. Noughts must take the centre: every other answer loses.

Crosses open in the centre. Any corner holds; an edge loses.

Against an edge opening, the centre is safe too. After that, block every threat, and before each move ask one question: where could crosses make two threats at once?

Forks: two threats at once

Here is what happens when noughts answer a corner with an edge. Crosses take the centre, which threatens the diagonal, so noughts must block in the far corner. Now crosses take the other corner on the left and threaten two rows at once: down the left side and along the other diagonal. Noughts can block only one.

Noughts' edge reply (2) lets crosses take the centre (3), force the block in the corner (4) and fork (5): crosses win on either dot.

To avoid forks, count your opponent's lines. A line with two of their marks is a threat you must block now. A line with one of their marks and none of yours is a threat in the making, and where two such lines cross at an empty square, a fork is one move away. Take that square yourself, or make a threat of your own that forces them to answer somewhere else.

The corner trap

This one catches noughts even after the right first answer. Crosses open in a corner, noughts take the centre, and crosses take the opposite corner. Now the natural-looking move — another corner — loses: it threatens the diagonal, crosses block it in the last corner, and that block is a fork.

Noughts take a corner (4); crosses block (5) and threaten two rows at once.

The answer is an edge. Crosses must block it, and their block makes no fork.

Against opposite corners, play an edge. After that, noughts' blocks come with threats of their own, and the game runs to a draw.

4×4: close the lines early

The 4×4 board is a draw too, but it is much easier to lose, because the threats take longer to show. There are only ten lines — four rows, four columns and two diagonals — and each needs all four squares. So one nought anywhere in a line kills it for crosses, and the whole game is a race to kill lines before they fill up.

The four corners and the four middle squares each lie on three lines, the edges on only two. Those eight squares are worth the most, for attack and for defence.
A line with two crosses and no nought is not yet a threat, but two such lines crossing at an empty square are a fork in waiting. Put a nought in one of them.
Watch the diagonals. They are the lines people forget, and a missed diagonal is the classic way to lose on 4×4.

Noughts left the diagonal and the third column with two crosses each. The ringed square makes three in both, threatening both dots, and noughts can block only one.

A move earlier, a nought on the ringed square, on C4 or on D4 would have held the draw. Every other move lost.

Five in a row: fours and threes

On the big board the words for what you have are about how many ways a row can still be finished:

a four is four marks that make five with one more move. A closed four, blocked at one end, has one winning square, and must be blocked there at once;
an open four, free at both ends, has two. It cannot be blocked: an open four wins;
an open three is three in a row with room on both sides, so that one more move makes an open four. It must be answered now, or it is too late.

An open four: two winning squares, and noughts can block only one.

A closed four: one winning square, and noughts must block it now.

Block an open three right up against it, at one end or the other. A block a square further away does not work: crosses add a mark at the other end and have an open four. Choose the end where your block also helps your own marks, or where crosses have more support nearby. With a broken three, such as two marks, a gap and one more, block in the gap.

An open three. Block on one of the ringed squares, right next to it — never a square further out.

Win the big board with a double threat

As on 3×3, a single threat is always blocked, so you win with two at once:

The 4-3. One move makes a four and an open three together. The four must be blocked, and the three then becomes an open four.
The 3-3. One move makes two open threes. Only one can be blocked in time.

The ringed square makes a four along the row and an open three down the column.

Noughts must block the four; the three becomes an open four and wins on either dot.

To get there, keep your marks together and near the middle. Marks a square or two apart share lines and support each other; a lone mark far away builds nothing, and a mark near the edge has fewer rows of five through it. Use fours as forcing moves: your opponent must answer where you choose, which buys you a free move for the next threat. And when you defend, block where the block also starts a line of your own.

Moving first on the big board

On 15×15 crosses have a real edge. For five in a row with these free rules, computers proved in the 1990s that crosses, playing perfectly, always win. Nobody plays perfectly, but between two good players crosses win more often. That is why the marks swap after every game at a friend's table.

As noughts, defend early: block open threes the moment they appear, and break up twos that are growing towards each other. Look for a four of your own whenever you can make one — a forced block is the only time crosses stop attacking.

How to beat the computer

Easy takes a win when it sees one, but lets many threats through. On 3×3, open in a corner and it will soon walk into a fork.
Medium blocks your rows, so a single threat will not do. Set up a fork: on 3×3 the corner openings above; on the big board a 4-3.
Hard never loses on 3×3; a draw is a good result there. On 4×4 it rarely slips either. On 15×15 it can be beaten, but only by a double threat it did not see coming: play crosses there to start with.

Play tic-tac-toe against the computer and try the traps on every level. If you are unsure of a rule, the full tic-tac-toe rules are one page.