Use the classic 3×3 rules
X moves first. Players alternate placing one mark in an empty square; an existing mark never moves. Three matching marks in a horizontal, vertical or diagonal line win immediately. A full board without a winning line is a draw. This guide uses 3×3 throughout: Happy Games also offers larger boards with different winning-line lengths.
For the examples, label columns a, b and c from left to right, and rows 1, 2 and 3 from bottom to top. The centre is b2, the top-left corner is a3 and the bottom-right corner is c1. These are the labels used in this guide and the website practice board.
A fork needs two different winning squares
Two marks in one open line create a single threat: the opponent can occupy its last square. A fork creates at least two such threats with different empty finishing squares. Blocking one then leaves the other available.
Before looking for a fork, check whether you can win now. Next, check whether the opponent can win on their next move. A planned double threat is too slow if they can end the game immediately. Count the empty finishing squares, not just the number of lines passing through a mark.
Build the fork: opposite corners and a missed defence
Start a fresh board and play X on a3, O on b2, then X on c1. X now holds opposite corners and O holds the centre. It is O's turn. Suppose O chooses the remaining corner a1. That is the mistake this example examines; it is not a forced reply.
X answers on c3. Besides blocking O's diagonal from a1 through b2, this creates two threats: X can complete the top row at b3, or the right column at c2. Both squares are empty.
| a | b | c | |
|---|---|---|---|
| 3 | X | · | X |
| 2 | · | O | · |
| 1 | O | · | X |
Why blocking one line is now too late
If O blocks b3, X plays c2 and completes c1–c2–c3. If O blocks c2, X plays b3 and completes a3–b3–c3. A move elsewhere stops neither threat, and O has no immediate winning move in this position.
The lesson is to notice the fork before X plays c3. Once two separate winning squares exist and you cannot win immediately, occupying only one of them does not solve the problem.
Defend earlier by choosing an edge
Return to X on a3 and c1, with O on b2 and O to move. Choose an edge instead of a corner: for example, O on b3. O now threatens b1 down the middle column. X has no immediate win, so X must play b1 to avoid losing on O's next turn.
This defence works because it makes an immediate threat while denying X a free move to build the fork. In this particular opposite-corners position, any of the four edge squares can preserve a draw with correct follow-up. This is a pattern to recognise, not a rule that every defensive move must go on an edge.
| a | b | c | |
|---|---|---|---|
| 3 | X | O | · |
| 2 | · | O | · |
| 1 | · | · | X |
Follow the defence all the way to a draw
After O b3 and X b1, do not stop calculating: X now threatens a1 along the bottom row. O plays a1. One complete continuation is X c3, O c2, then X a2. The board is full and neither player has three in a line.
The full sequence is X a3 → O b2 → X c1 → O b3 → X b1 → O a1 → X c3 → O c2 → X a2. It illustrates why a good defensive move still needs careful follow-up; it does not automatically secure the result by itself.
Practise both decisions on the website
The guided example below begins with a different position: X can complete a diagonal in one move. Finish it, then select “New 3×3 game” for an empty local board. Two people can take turns on the same device, or you can place both sides yourself to replay the sequences above.
First try O a1 and see why either block loses. Restart, try O b3 and follow the drawing line. Before each move, ask: can I win now, must I stop an immediate win, and which move could create two separate winning squares? The local practice does not create an online match; to play in Telegram, open Happy Games and choose Tic-Tac-Toe.