sudokUI › Sudoku techniques › Finned X-Wing
Finned X-Wing in sudoku
Two rows (or columns) have candidate X at all four corners of a rectangle, and their only other candidates for X, the fins, all lie in one corner's box.
Why it works
If no fin is true, a plain X-Wing removes X elsewhere in its columns; if one is, its box holds no other X. Cells of those columns in the fin box, outside the two rows, lose X, or likewise with rows and columns swapped.
How to spot it
Look for two rows or columns that would form an X-Wing but for extra candidates, all inside the box of one corner.
Worked example
Finned X-Wing on 8: in columns 3, 9, 8 sits in rows 5, 8 (r5c3, r8c3, r5c9, r8c9) except for the fin r4c3. Either the fin is false and the fish holds, or the fin is true; a 8 that sees every fin and lies in the cover rows outside the base is false both ways. So 8 is removed from r5c1.
Practise it on a real puzzle. sudokUI generates a puzzle that needs Finned X-Wing and takes you straight to the position where it applies. Hints draw the pattern on the board and explain the step.
In a puzzle's rating
Each time the solver needs Finned X-Wing, the puzzle's difficulty rating grows by 130. Its class is Hard. It is needed in 1 in 55 puzzles that sudokUI generates. How the rating works.
More finned fish
Sashimi X-Wing · Finned Swordfish · Sashimi Swordfish · Finned Jellyfish · Sashimi Jellyfish