DECISION STUDY · four-08
Prove the fork against each reply
Orange to move. Find a move after which every legal opposing reply leaves you an immediate winning move.
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Use the column buttons to drop a token. Orange (O) and Violet (V) take turns. You control both sides here; there is no computer opponent. Test a reply, then undo to compare another.
Keyboard: Tab to an enabled control, then press Enter or Space. The study starts from the diagram shown; reload can restore your own saved moves.
Try the task before inspecting the answer.
Show the starting-position answer and evidence
Forced next-turn winning columns: 3, 4. For each qualifying move, every legal reply has been checked, including replies that might win immediately for the opponent. The reply table gives your winning response.
| First move | Immediate effect | Checked consequence |
|---|---|---|
| Column 2 | Lands at R2C2. | No opponent win in one move; no forced next-turn win established. |
| Column 3 | Lands at R4C3. | Forces a win on your next turn against every legal reply. |
| Column 4 | Lands at R3C4. | Forces a win on your next turn against every legal reply. |
| Column 6 | Lands at R5C6. | No opponent win in one move; no forced next-turn win established. |
| Column 7 | Lands at R2C7. | No opponent win in one move; no forced next-turn win established. |
After column 3: every opposing reply
| Opponent drops in | Your immediate winning response |
|---|---|
| Column 2 | Columns 3, 4 |
| Column 3 | Column 4 |
| Column 4 | Column 3 |
| Column 6 | Columns 3, 4 |
| Column 7 | Columns 3, 4 |
After column 4: every opposing reply
| Opponent drops in | Your immediate winning response |
|---|---|
| Column 2 | Column 3 |
| Column 3 | Column 3 |
| Column 4 | Column 3 |
| Column 6 | Column 3 |
| Column 7 | Column 3 |
How this position was checked
Reached by the recorded alternating legal column drops from an empty board.
Reproduce from the regular starting board: 7, 4, 1, 5, 6, 2, 5, 7, 2, 5, 1, 1, 1, 5, 1, 7, 3, 2, 5, 4, 2, 1, 4, 5, 7, 3 (column numbers).
Explain what you learned
Choose one first move from the comparison table. Explain its effect without rereading the answer. Then change the first move and identify which part of the explanation changes. These comparisons answer the stated task from this starting position. They do not prove the best move or eventual winner in a different position.
Rules and limits
Two players share a six-row, seven-column board. Orange starts, then Orange and Violet alternate dropping one token into a non-full column. It lands in the lowest empty square. Four or more consecutive tokens of one player horizontally, vertically or diagonally wins. A full board without a winner is a draw. This classic version has no token removal, wrapping or free placement above a gap. Row 1 is the top and row 6 the bottom.
An immediate win can be checked exactly by making a legal drop and inspecting all lines. A forced defensive move or fork claim must also check the opponent's immediate winning replies. A local threat calculation is not a full-game solver or a promise of perfect play. Statements about central control or future shapes are strategic guidance unless a complete response analysis is supplied. The regular game has two local human players, not a computer opponent.