Tic Tac Toe Strategy: Never Lose on 3 × 3

The rules

X goes first. Players take turns marking an empty square. The first to make a line of three (across, down or diagonally) wins; a full grid without a line is a draw. On the 5 × 5 grid a line is four in a row. Against the robot you choose whether to play first or second; online, the server decides who starts.

The priority list

On your turn, do the first of these that applies:

1. Win: if you can complete a line, do it.
2. Block: if the opponent could complete a line next move, block it.
3. Fork: make a move that creates two lines of two at once (two ways to win next move). The opponent can block only one.
4. Block a fork: if the opponent could fork next move, stop it, preferably by making a two-in-a-row that forces them to block instead (as long as the forced block does not give them a fork).
5. Centre: take it if free.
6. Opposite corner: if the opponent is in a corner, take the corner diagonally opposite.
7. Any corner.
8. Any side.

Follow this list and you will never lose on 3 × 3.

Openings as X

Open in a corner or the centre. A corner opening gives the opponent the most ways to go wrong: only a centre reply holds the draw, and many players reply in another corner or a side, which loses to a fork. Opening in the centre is also sound; the opponent must reply in a corner.

Replies as O

• X in a corner: take the centre. Anything else loses to correct play.
• X in the centre: take a corner. A side reply loses.
• X on a side: take the centre; corners also hold, but the centre is simplest.

Forks: how to see them

A fork is a square that lies on two lines in which you already have one mark and the opponent has none. Corners lie on three lines, the centre on four, sides on two, which is why corners and the centre make forks and sides rarely do. Before each move, look at the empty squares your marks share lines with; if two of your lines cross at an empty square, that square is a fork.

Playing the Hard robot

The Hard robot never makes a losing move on 3 × 3. Use it to drill the priority list: play X and aim to draw every game; play O and hold the draw against each opening. When every game is a draw without effort, you have learned the game. Easy and Normal leave openings and can be beaten.

The 5 × 5 grid

Four in a row on 5 × 5 is a much more open game. The centre square matters even more, since more lines of four pass through it. Three in a row with both ends open is already a win (two squares to complete it, one block), so block an open-ended three the moment it appears, and build your own. Forks work as on 3 × 3 but with more lines to combine, and the Hard robot searches ahead rather than knowing the answer, so wins are possible, especially as the first player.

Common mistakes

1. Replying to a corner opening with another corner. It loses to a fork.
2. Blocking a line when you could win. Always check your own win first.
3. Ignoring forks. Most 3 × 3 losses are forks seen one move late.
4. Playing sides early. Sides lie on only two lines.
5. On 5 × 5, blocking one end of an open three. It is already lost; block it before it opens at both ends.

Glossary

• Fork: a move that creates two winning threats at once.
• Open three (5 × 5): three in a row with empty squares at both ends.
• Perfect play: moves that never lose; on 3 × 3 they always draw.
• Centre, corners, sides: the three kinds of square, lying on four, three and two lines.