Step 15 · Expert
What is XY-Wing? How to Solve Killer Sudoku
Three cells with two candidates each — a pivot XY and two pincers XZ and YZ that each see the pivot — let you remove Z from any cell that sees both pincers.
What it is
Built from three cells with two candidates each.
• The pivot: 1 or 2
• Cell A (sees the pivot): 1 or 3
• Cell B (sees the pivot): 2 or 3
If the pivot is 1, A is 3. If the pivot is 2, B is 3. So A or B is always 3.
That means any cell that sees both A and B can't be 3.
How to spot it
Start from a two-candidate cell and look for two more two-candidate cells it sees. Three cells, three digits, each pair sharing one digit.
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.
R4C8 can only be 6 or 7. If it's 6, R6C7 becomes 4. If it's 7, R2C8 becomes 4. So one of R6C7 and R2C8 is always 4, and any cell that sees both (R2C7, R5C8, and R6C8) can't be 4.