Line Three: defend opposite corners with a forcing move.
See why an edge can stop a corner trap, follow the forced replies to a draw, and test a rotated position.
You take two opposite corners in Line Three. The computer has the center, and its next mark goes on an edge. That choice can look weaker than taking another corner. In this position, however, the edge is the move that keeps a draw available.
The useful idea is to defend against a future fork by creating an urgent threat of your own. We will compare an unsafe corner with a safe edge, then follow the edge continuation to the end. You play X in the live game; O is the computer. The explanation temporarily considers O’s decision so you can understand the reply you receive.
Read the position before choosing a move
Start with X at top-left and bottom-right, and O in the center. O is next. A dot is an empty square. You can reach this opening by playing top-left, letting O take the center, then playing bottom-right.
X · · · O · · · X
Neither side can win immediately. The diagonal between the X corners is already blocked by O. Yet X can combine a third corner with its existing marks to create two different winning squares. O needs to address that future possibility before it becomes an unavoidable loss.
The corner reply that allows the trap
Suppose O chooses top-right. That adds an O threat along the other diagonal: bottom-left would complete it. X answers at bottom-left, blocking that diagonal and building its own fork at the same time.
X · O · O · X · X
Now X can win at middle-left through the left column, or at bottom-middle through the bottom row. O has no immediate winning move. If O blocks middle-left, X finishes the bottom row. If O blocks bottom-middle, X finishes the left column. The other O replies, top-middle and middle-right, leave both X winning squares open. O cannot cover two different squares with one mark.
This is why creating a threat is not enough on its own. O’s top-right move did threaten a win, but the required X block was also the move that created the fork. A forcing move must be checked all the way through the forced reply.
The edge reply that changes X’s next decision
Restore the original opening on paper. This time put O at top-middle:
X O · · O · · · X
O now threatens bottom-middle through the middle column. X has no winning move this turn. To avoid an immediate loss, X must take bottom-middle. That reply does not produce two separate X winning squares: it produces one, at bottom-left.
X O · · O · · X X
O must still defend. Taking bottom-left stops the X row and creates an O diagonal threat at top-right. Stopping a fork is a useful step, not permission to ignore the next single threat.
What if X tries to fork instead of blocking?
After the top-middle O move, X could choose bottom-left. That would create two X winning squares, at middle-left and bottom-middle. But O can immediately win at bottom-middle before X gets another turn. The threatening shape exists; it does not force a win because O’s current win takes priority.
There are five X replies to the top-middle O move. Bottom-middle is the only one that blocks O’s middle-column win. None of the other four gives X an immediate win. This is the precise sense in which the reply is forced: X must choose it to avoid losing on O’s next move.
Finish one complete defensive continuation
From the diagram where X has blocked at bottom-middle, follow these moves. Each player addresses the next immediate threat:
- O takes bottom-left: blocks X’s bottom row and threatens top-right along the diagonal through the center.
- X takes top-right: blocks that diagonal and threatens middle-right through the right column.
- O takes middle-right: blocks that column.
- X takes middle-left: fills the last empty square. No winning line exists.
X O X X O O O X X
Check all three rows, three columns, and both diagonals. Every line contains both marks, so the full board is a draw. The edge defense preserves this result against careful play. It does not guarantee that every later move will be safe, and it does not force a win for O.
Compare all six opening replies
The starting diagram has six empty squares. Evaluating every legal continuation gives two groups:
- Top-middle, middle-left, middle-right, or bottom-middle: each edge reply lets O secure a draw with correct later defense. It makes a threat through the center that X must answer.
- Top-right or bottom-left: either remaining corner lets X force a win by occupying the other corner and creating the fork.
This statement belongs to the exact opening above. “Edges are always best” would be a poor general rule. On another board, an edge may lose immediately, and a corner may be the only winning or blocking move. Scan the occupied lines first.
Self-check: rotate the idea
Here, X has the other pair of opposite corners. O has chosen middle-left beside its center mark. It is X’s turn. Which square must X take, and what must O do afterward?
· · X O O · X · ·
Show the two required replies
X must take middle-right to stop O completing the middle row. There is no immediate X win elsewhere. After that block, X has top-right and middle-right, so it threatens bottom-right. O must take bottom-right next. Rotating a board changes the square names, but checking the actual lines gives the same defensive reasoning.
Apply the lesson in a live round
In the live game, the computer can use this edge defense after you take opposite corners. You remain X; there is no control for switching sides or loading these diagrams. The unsafe O corner branch is a paper comparison, and a careful computer need not allow it.
Before treating a move as a fork, ask three questions: does the other side already have a win, which reply stops it, and what threats remain after that reply? This prevents you from calling a promising two-threat shape a forced victory when the opponent can finish first.