The W-Wing Technique in Sudoku
The W-Wing is a neat pattern that connects two identical pairs across the grid. It is less famous than the XY-Wing but just as satisfying to spot.
The shape
A W-Wing uses two cells that hold the same two candidates, say A and B, and that do not see each other. Between them there is a strong link on one of the values. That means somewhere on the board B appears in exactly two cells of a unit, and each of those two B cells lines up with one of your pair cells.
The elimination
Because the strong link forces one end or the other to be B, one of your two pair cells is pushed to be A. So at least one of the pair is A no matter what. Any cell that sees both pair cells therefore cannot be A, and you erase A from it.
How to hunt for one
- Find two cells with the same two candidates that do not share a unit.
- Pick one of the two values and look for a unit where it has exactly two candidate cells.
- Check that those two cells each see one of your pair cells.
- If they do, erase the other value from any cell seeing both pair cells.
Why it is worth knowing
The W-Wing reaches eliminations that pairs and simple scanning miss, and its shape is clean once you have seen a few. It sits comfortably beside the XY-Wing in your toolkit, and many stuck boards yield to one or the other.
Ready to put it into practice?
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