Find two cells that can only be x or y and don't see each other. If a row, column or box has x in just two cells, one seeing each of them, then one of the two cells must be y. No cell that sees both can be y.
W-Wing connects two identical two-option cells that are far apart, using a single digit as the bridge.
It is harder to spot than an XY-Wing because the link is a digit rather than a cell, but it often clears several candidates at once.
A W-Wing
Where the pattern lives
The pattern
Candidates removed
Solved as a result
R4C5 and R6C3 can only be 3 or 9. In row 9 the 3 fits in just two cells, R9C5 and R9C3. If R4C5 were 3, R9C5 couldn't be, so R9C3 would be the 3 and R6C3 would have to be 9. So one of R4C5 and R6C3 is the 9, and the 9 can be removed from every cell that sees both of them: R4C1. That leaves R4C1 with only one candidate: 3.
This position comes from a puzzle generated on this site, after every single has been filled in. Boxes are numbered 1–9 from the top left, row by row; R5C4 means row 5, column 4.
How to spot it
1Find two cells with the same two candidates, x and y, that don't share a row, column or box.
2Look for a row, column or box where x has exactly two places.
3Check that one of those places sees the first cell and the other sees the second.
4Remove y from every cell that sees both two-option cells.
5Try it with x and y swapped as well.
Why it works
Suppose the first cell is x. The bridge cell it sees can't be x, so the bridge's other cell is x, and that one sees the second cell, which then can't be x: it is y.
If the first cell isn't x, it is y.
Either way one of the two cells is y, so a cell that sees both can't be y.
Common mistakes
Two-option cells with different pairs. Both must be exactly x and y.
A bridge where x has three places. It must be exactly two.
Removing x instead of y. The bridge digit is x; the one you remove is the other.
Questions
Why is it called a W-Wing?
The two cells and the bridge between them, drawn on the grid, look a bit like a W.
Do the two cells have to be in different boxes?
They must not see each other at all, so they are in different rows, columns and boxes.
Is there anything after W-Wing?
Yes: coloring and longer chains. When no pattern is left, our hints still prove the answer by contradiction, showing what goes wrong with each other option.
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