W-Wing
Two bivalue cells holding the same {X, Y} candidates, connected by a strong link on Y. Any cell that sees both endpoints loses X.
Advanced sudoku technique
What it is
A W-Wing connects two bivalue cells via a third unit. Both endpoint cells have the same two candidates {X, Y}. Inside a third unit (row, column, or box) somewhere between them, digit Y has a strong link — exactly TWO candidate cells. One end of the strong link sees endpoint A; the other end sees endpoint B. The deduction: if endpoint A is Y, the strong-link end touching A is forced NOT to be Y, so the other end of the strong link IS Y, which means endpoint B sees a Y and must be X. By symmetry, if endpoint B is Y, endpoint A must be X. Either way, at least one endpoint is X. Therefore X can be eliminated from every cell that sees BOTH endpoints.
When to use it
On expert and master puzzles when XY-Wing and XYZ-Wing don't yield a move. Hunt for pairs of bivalue cells sharing the same candidate set plus a strong link on one of those candidates somewhere between them.
Worked example
Endpoint A R2C3 = {4, 7} and endpoint B R7C8 = {4, 7}. In column 6, digit 7 has only two candidate cells: R2C6 (seeing endpoint A via row 2) and R7C6 (seeing endpoint B via row 7). That's a strong link on 7. By the chain above, one of the endpoints must end up as 4. Therefore 4 can be eliminated from any cell that sees BOTH R2C3 and R7C8.
Try it
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