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.
Want the full theory?
The W-Wing guide on Sudoku247Wiki walks through the logic step by step, with worked examples, diagrams and FAQs.
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