Tic Tac Toe traps and forks: how to set them and how to block them
When you have two ways to win and your opponent can block only one, the game is over. That's a fork, and nearly every decisive game of Tic Tac Toe ends with one.
What is a fork?
A fork is a single move that creates two lines that each need just one more mark. Your opponent gets one move, blocks one line, and you complete the other. Against a player who never misses a simple block, a fork is the only way to win.
In the diagram, X threatens the left column (middle-left square) and the anti-diagonal (top-right square). O can't cover both.
The key idea: forced moves
You rarely get a fork for free. The usual recipe is to make a single threat first. Your opponent has to block it, which means you choose where their next mark goes. If you've picked the threat well, their forced block does nothing for them, and your next move is the fork. Think of it as a two-step combination: threat, then fork.
Pattern 1: corner, centre, corner
X opens in a corner and O replies on an edge, a common beginner mistake.
After step 3, X threatens both the left column and the diagonal from bottom-left to top-right. O can block only one.
Pattern 2: the centre opening against an edge reply
When X opens in the centre, O must answer in a corner. Here's what happens after an edge reply:
It's the same final shape as pattern 1, reached by a different move order. Corners and the centre working together create most forks.
Pattern 3: three corners
X opens in a corner, O takes a corner too.
X now threatens the bottom row and the main diagonal through the centre. The lesson for the second player: when X opens in a corner, take the centre, not a corner.
Pattern 4: the forced block that is also a fork
This is the most beautiful trap in the game, because the defender walks into it while making a perfectly reasonable move.
O's corner move looked active because it made a threat. But X's forced block lands on the last corner and creates two threats at once. The correct move for O was an edge, which makes a threat X must answer without creating anything for X.
A trap for the first player
Forks work in both directions. If X opens on an edge, O takes the centre, and X plays the edge opposite, O can turn the tables:
X's block created a threat along the bottom row, but O's reply both stops it and creates two threats of its own: the left column and the diagonal.
How to block a fork before it happens
Once a fork is on the board, it's too late. You have to stop it a move earlier. There are two ways:
- Occupy the fork square. If there's only one square where your opponent could fork, take it yourself.
- Make a threat of your own. If there are two possible fork squares, you can't occupy both. Instead, make two in a row so your opponent is forced to block, and choose a threat whose blocking square is not a fork square for them.
The second technique is exactly why O plays an edge in pattern 4. The edge creates a line through the centre, X is forced to block on the opposite edge, and that square gives X no fork.
Before every threat you make, look at the blocking square. If your opponent's forced block gives them two in a row, or worse, a fork, find a different move.
Practising forks
The fastest way to learn these patterns is to play them over and over. In the TicTacToe app, the Medium opponent always takes a win and always blocks a single threat, but it doesn't look ahead to forks. That makes it the perfect sparring partner: you can only beat it with the patterns on this page. Then switch to Hard, which never falls for any of them, to practise your defence.