Tic Tac Toe strategy: can you always win or draw?

01The short answer: can you always win or draw?

You can always draw, but you cannot always win. Tic tac toe on the 3x3 board is a solved game. If both players play perfectly, every game ends in a draw, whoever moves first. The first player, X, cannot force a win against a perfect opponent, and the second player, O, can always hold the draw as long as they answer correctly.

That is not the end of the story, because most people do not play perfectly. If you know the right moves you will never lose, and every time your opponent makes a small mistake you get a chance to win. This guide shows you both halves: how to stay safe, and how to set traps. You can try everything below in our free Tic Tac Toe, including bigger boards.

02Why is tic tac toe a draw?

The game is small enough to check completely. There are 255,168 possible games, counting every sequence of moves that ends in a win or a full board, and 5,478 distinct legal positions. If you treat rotations and reflections of the board as the same, there are only a few hundred positions that matter. A computer can examine all of them in a moment, and when it does, it finds that every perfect line ends in a draw.

The Hard computer on the 3x3 board does exactly that, so it never loses. If you want a real test of your strategy, try to draw against it. That is a good result, and it means you have learned the game.

03The eight-step rule list for X and O

Here is a simple order of priorities. Work down the list on every move and play the first one that applies. This is close to the rule set that the researchers Allen Newell and Herbert Simon wrote into an early tic-tac-toe program, and it will keep you from losing.

• 1. Win. If you have two in a row and the third square is empty, take it.
• 2. Block. If your opponent has two in a row, take the third square.
• 3. Fork. Make a move that creates two separate threats at once.
• 4. Block a fork. If your opponent could make a fork next turn, prevent it.
• 5. Centre. Take the middle square if it is free.
• 6. Opposite corner. If your opponent holds a corner, take the opposite one.
• 7. Empty corner. Take any free corner.
• 8. Empty side. Take an edge square only when nothing else is left.

04Number the squares so we can talk about moves

To describe positions we number the squares 1 to 9 from left to right and top to bottom. Squares 1, 3, 7 and 9 are the corners, square 5 is the centre, and 2, 4, 6 and 8 are the edges. On the keyboard version of the game, the number keys 1 to 9 place a mark on the matching square.

05The best opening for X: take a corner

Each of the three kinds of opening is safe for X, but they are not equally good at setting problems. A corner opening is the strongest in practice. After X takes a corner, O has eight possible replies, and exactly one of them keeps the draw: the centre. Every other reply loses with best play by X. Many beginners reply with an edge or another corner, and that gives X a forced win.

A centre opening is also safe. O must reply in a corner. An edge reply loses. An edge opening is the weakest for X: O can draw in several ways, and it gives X the least to work with, so avoid it unless you want a quiet game.

06How to win as X: the classic fork

A fork is a move that makes two threats at the same time, so your opponent can only stop one. Here is the most common way it happens. X takes corner 1. O makes the mistake of replying on an edge, square 2. X takes the centre, 5, which threatens to complete the diagonal 1, 5, 9. O must block at 9. Now X plays corner 7. That makes two threats at once: 1, 4, 7 down the left side needs square 4, and 3, 5, 7 along the other diagonal needs square 3. O can block only one, and X wins with the other.

The lesson is to look for a corner and the centre together, with a third mark that sits on two lines at once. Whenever you have two marks and an empty square that is on two lines with them, you may have a fork.

07How to play O: draw against a good X

As O your job is to survive, and to wait for a slip. The replies are simple. If X opens in a corner, take the centre. If X opens in the centre, take a corner. If X opens on an edge, take the centre, or a corner next to it.

After that, keep to the rule list. A frequent trap is X taking two opposite corners after O has the centre. For example X has 1 and 9, and O has 5. If O now plays a corner, say 3, X is forced to block at 7, and that block makes a fork for X. O must instead play an edge such as 2, which forces X to respond, and the game settles into a draw. Remember this one: with X on two opposite corners and O in the centre, O plays an edge, never a corner.

08How to beat a weaker opponent

You can win against many people simply by being the one who sets problems. Start in a corner and see whether they answer in the centre. If they do not, you have a forced win and a fork to find. If they do, play the opposite corner and see whether they answer with a corner or an edge. If they answer with a corner, you win. If they answer with an edge, the draw is back, so shake hands and play again.

Against the Easy and Medium levels of the computer, a corner opening and a careful search for a fork usually wins quickly. Medium always blocks an immediate threat and takes an immediate win, but it does not plan for forks, so that is where you beat it.

09What changes on 4x4 and 5x5 boards?

Bigger boards keep the same ideas with a longer line to make. On a 4x4 board you need four in a row, and on a 5x5 board you can play for four or five. Three things change.

• An open three is a serious threat. With three in a row and both ends empty, a player has two ways to finish, so it must be blocked at once.
• Forks are much easier to build, because there is more space. The player who takes the centre and keeps adding marks that work in two directions usually pulls ahead.
• Blocking is not enough. On a small board you can win by blocking everything. On a large board you must build your own lines while blocking, or the other player will eventually get a double threat.

10Tic tac toe for kids and classrooms

Tic tac toe is a gentle first strategy game. It teaches turn-taking, noticing patterns and thinking one move ahead. A fun classroom exercise is to ask the class to predict the computer's reply before the move is played, or to let two pupils play while the rest of the class suggests moves.

For children who find the 3x3 game too easy, move to Medium, which they can still beat with a fork, and then to the bigger boards. The game also leads naturally into a talk about how computers solve games by checking every possibility.

11Mistakes to avoid

• Opening on an edge as X. It gives the other player the easiest game.
• Replying to a corner opening with anything except the centre.
• As O, answering two opposite X corners with another corner instead of an edge.
• Making two in a row without checking that the third square is actually empty and your opponent cannot just block it for free.
• Ignoring a fork that is one move away, because you are busy making your own line.

12Play Tic Tac Toe now

Reading about strategy is useful, but a few games teach it faster. Open Tic Tac Toe, pick the 3x3 board and try the corner opening against Medium. When you can beat Medium and draw Hard, try the 4x4 and 5x5 boards, or send a friend a link to play online. It is free and there is nothing to download. If you like the idea of lining up marks in a grid, try Connect Four next, where gravity adds a new twist.