Line Three: look for threats before opportunities.
Read urgent threats first, then verify a real two-way fork by following each possible reply.
There are only nine squares, but choosing a promising-looking one is not enough. A move can build a future threat while ignoring an immediate loss. A dependable way to read the board is to check urgent outcomes first and positional opportunities second.
First, take a win that already exists
Scan the three rows, three columns, and two diagonals. If any line contains two of your X marks and one empty square, filling that square wins. You do not need to block an opponent’s threat if your own move ends the game first.
X X · O O · · · ·
In this position, X should play the top-right square. Blocking O at the middle-right square would miss an immediate win. A checklist that says “always block” is incomplete; winning this turn comes first.
Second, prevent an immediate loss
If you have no winning move, find any line with two O marks and an empty square. Occupy that empty square. For example, when O has both upper corners and the top-middle square is empty, the middle square is urgent regardless of how attractive the center may seem.
If O has two distinct winning squares, one blocking move may not be enough. That usually means an earlier move allowed a fork. Reviewing the previous turn helps you find the cause rather than blaming the final forced position.
Third, understand a fork precisely
A fork is a move that creates two separate ways to win on the following turn. Here is a position with X to move. Consider the lower-left square.
Before X moves:
X · · · O · · O X
After X takes lower-left:
X · · · O · X O X
The new X creates one threat in the left column: middle-left would complete three X marks. The diagonal from lower-left to upper-right is blocked by O in the center. The bottom row is blocked by O as well. This move creates only one immediate threat, so it is not a fork. Worse, O can win immediately at top-middle, completing the middle column. X should have blocked that square instead of building its own threat.
Counting open lines explicitly is better than remembering a vague slogan such as “opposite corners always create a fork.” The exact occupancy matters.
A genuine two-threat example
Here is a different position with X to move. Each side has two marks, and neither side has already won. The dot marks an empty square.
X · O · · X O · ·
X has no immediate winning move. O threatens to complete the diagonal from top-right to bottom-left by taking the center. X must address that threat. Placing X in the center both blocks O and creates a fork:
X · O · X X O · ·
Now count the empty winning squares, not just the lines that contain X. Middle-left completes the middle row; bottom-right completes the diagonal from top-left through the center. Those are exactly two different winning squares. O has no immediate winning move that could end the game first.
Check every possible reply
Only four squares remain empty. Trace O’s options before assuming the fork works:
- If O takes middle-left, X wins at bottom-right along the diagonal.
- If O takes bottom-right, X wins at middle-left along the middle row.
- If O takes top-middle, both X winning squares remain available.
- If O takes bottom-middle, both X winning squares remain available.
One O move cannot cover two different squares, and O has no immediate win of its own. X can therefore win on the following turn whichever reply O chooses. That is a guarantee for this exact position under these rules, not a general promise that every corner or center move wins.
Test the reason, not just the shape
Why does the earlier lower-left move fail while this center move succeeds?
The earlier move leaves only one open X threat and ignores O’s immediate win at top-middle. The center move in the new example removes O’s immediate threat, then leaves two separate X winning squares. The difference is the occupancy and urgency of the lines, not whether the move looks like a familiar pattern.
Why count different winning squares instead of only counting promising lines?
A line must have two of your marks, one empty square, and no opponent mark to be an immediate threat. If several lines rely on the same empty square, the opponent may stop all of them by occupying that one square. In the worked fork, middle-left and bottom-right are different squares, so a single blocking move cannot stop both.
These are constructed teaching positions. The live computer may avoid the earlier choices needed to reach them. Use the diagrams to check the reasoning, then apply the same win–block–fork order to the positions that actually appear in your game.
What to expect from our opponent
The computer searches legal continuations and avoids losing when it can. With careful play from both sides, draws are expected. Use each round to practice checking every line and identifying the reason for a move. You do not have to beat the computer for the exercise to be useful; a well-defended draw means you prevented its threats.