Find a cell with exactly two candidates — call them X and Y. This is the pivot.
XY-Wing
Three two-candidate cells that trap a digit, whichever way the pivot goes.
The XY-Wing (also called the Y-Wing) uses three cells that each have exactly two candidates. The pivot holds {X,Y}. One wing, which shares a unit with the pivot, holds {X,Z}; the other wing, also seeing the pivot, holds {Y,Z}. If the pivot is X, the first wing must be Z; if the pivot is Y, the second wing must be Z. Either way, one wing is Z — so Z can be removed from any cell that sees both wings.
Worked example
12
3
3
13
23
3
3
3
How to apply it
Look for a two-candidate cell the pivot can see that holds X and a new digit Z.
Look for another two-candidate cell the pivot can see that holds Y and the same Z.
Make sure the two wings are not the same cell and do not both contain X or both contain Y.
Remove Z from every cell that sees both wings.
When to use it
The XY-Wing is one of the most useful expert-level techniques because two-candidate cells are common late in a hard solve. On Sudoku.tips, many expert puzzles need it at least once.
Frequently asked questions
Is the XY-Wing the same as the Y-Wing?
Yes, the two names describe the same pattern. Its close relative, the XYZ-Wing, uses a pivot with three candidates and needs a target cell that sees all three cells.