These five original puzzles connect familiar 3×3 play with our four-line rules on 4×4 and 5×5. Identify the size and player to move first. Leave answers collapsed until you have tested your own solution.
In this guide
3×3: defend and fork together
X has a1 and c3; O has b2 and a3. X is to move. Find a move that removes the immediate danger and prepares two winning cells. The study image does not pre-place the answer: c1 is initially empty.
Position as text
X: a1, c3. O: b2, a3. To move: X. Rows count from the top.
Show hint
First inspect O’s a3–b2–c1 diagonal. Which empty cell stops it?
Answer and explanation
X plays c1, blocking O while creating finishes at b1 and c2. After O takes either, X wins at the other. O has no immediate win in this position.
4×4: connect the crossing lines
X has b1, a2, c2 and b3. O has a1, d1, d3 and a4. X is to move. Three marks alone do not win here. Instead find a fork with two distinct finishing cells.
Position as text
X: b1, a2, c2, b3. O: a1, d1, d3, a4. To move: X. Rows count from the top.
Show hint
Row two and column b cross at an empty square.
Answer and explanation
b2 connects both plans. X can then finish at d2 in row two or b4 in column b. The targets are empty and different; O can cover only one and cannot win immediately here.
5×5: can one block stop the open three?
X occupies b3, c3 and d3; O occupies a1 and c1. O is to move. No four-line has formed yet. Can one ordinary blocking move stop both X finishes?
Position as text
X: b3, c3, d3. O: a1, c1. To move: O. Rows count from the top.
Show hint
Write down both four-cell windows in row three.
Answer and explanation
No. a3 completes a3–b3–c3–d3, while e3 completes b3–c3–d3–e3. Both endpoints are empty. O can take only one and has no winning move here; X wins at the other end next.
5×5: look beyond adjacent threes
X has a4, b4 and d4; O has b1, d1 and c2. X is to move. Which single square ends the game immediately? Also explain why e4 does not do the same.
Position as text
X: a4, b4, d4. O: b1, d1, c2. To move: X. Rows count from the top.
Show hint
Four consecutive marks means no intervening space.
Answer and explanation
c4 closes the gap to form a4–b4–c4–d4. e4 leaves c4 empty, so neither a4–d4 nor b4–e4 is complete. An extra mark elsewhere in the row cannot replace the missing cell of the window.
5×5: the offset diagonal
X has b1, c2 and d3; O has a1, e1 and a5. X is to move. Although O occupies a1, find an immediate diagonal win for X.
Position as text
X: b1, c2, d3. O: a1, e1, a5. To move: X. Rows count from the top.
Show hint
Start the line at b1 rather than a corner.
Answer and explanation
e4 completes b1–c2–d3–e4, offset from the long main diagonal. O at a1 lies outside this four-cell window and does not block it.
Go beyond finding the answer
Do not mentally rotate the diagram without rotating its coordinates too. After replaying a puzzle, take the opponent’s perspective: which replies are legal, which lose immediately and which genuinely block? These diagrams are not importable saved positions, but the short studies can be recreated in a fresh two-player round on one device.