Step 25 · Master
What is Finned X-Wing? How to Solve Killer Sudoku
An X-Wing with a few extra candidates (the fin) near one corner: you can still remove the digit from cells that are in the X-Wing's columns and in the fin's box.
What it is
An almost-X-Wing. Near one corner, inside the same box, a few extra candidates stick out. That's the fin.
• If the fin is the answer: the other cells in the fin's box can't be the digit.
• If it isn't: it's a normal X-Wing.
Either way, cells in the fin's box that line up with the X-Wing's column can't be the digit.
How to spot it
When something looks almost like an X-Wing, check whether the extra candidates are all in one box.
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.
Think about 1 only. column 2 and column 8 almost form an X-Wing (R7C2, R8C2, R7C8, and R8C8), but extra candidates stick out at R9C8 (the fin). If the fin is 1, the other cells of box 9 aren't 1. If it isn't, it's a normal X-Wing on row 7 and row 8. Either way, R7C9 can't be 1.