Step 21 · Master
What is Finned X-Wing? How to Solve 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
Yellow cells are where to look, orange cells are the reason, red digits are candidates you can remove, and the blue digit is the answer.
Think about 6 only. column 4 and column 3 almost form an X-Wing (R2C4, R5C4, R2C3, and R5C3), but extra candidates stick out at R3C3 (the fin). If the fin is 6, the other cells of box 1 aren't 6. If it isn't, it's a normal X-Wing on row 2 and row 5. Either way, R2C1 can't be 6.