Step 16 · Expert
What is XYZ-Wing? How to Solve Killer Sudoku
An XY-Wing whose pivot has three candidates (XYZ): you can remove the shared digit Z from any cell that sees the pivot and both pincers.
What it is
A cousin of the XY-Wing where the pivot has three candidates (1, 2, 3). A is 1 or 3, and B is 2 or 3.
If the pivot is 1, A is 3. If it's 2, B is 3. Otherwise the pivot itself is 3.
One of the three cells is always 3, so any cell that sees all three can't be 3.
How to spot it
Start from a three-candidate cell and look for two cells it sees, each holding two of its candidates.
Example
Dotted outlines are cages, with their total in the top-left corner. Yellow cells are where to look, orange cells are the reason, red digits are candidates you can remove, and the green digit is the answer.
R2C7 can be 5, 6, or 3. If it's 5, R2C3 is 3; if it's 6, R2C2 is 3; otherwise R2C7 itself is 3. One of the three is always 3, so any cell that sees all three (R2C5 and R2C9) can't be 3.