sudokUI › Sudoku techniques › Finned Swordfish
Finned Swordfish in sudoku
Three rows (or columns) have candidate X only in three shared columns (or rows) plus fins, extra cells all in one box, with at least two non-fin candidates per row.
Why it works
If no fin is true, a plain Swordfish 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 three rows, lose X, or likewise with rows and columns swapped.
How to spot it
When three rows almost form a Swordfish, check whether every extra candidate for X sits in one box crossed by one of its three columns.
Worked example
Finned Swordfish on 3: in rows 2, 3, 7, 3 sits in columns 2, 4, 7 (r2c2, r2c4, r3c4, r3c7, r7c4, r7c7) except for the fin r7c3. Either the fin is false and the fish holds, or the fin is true; a 3 that sees every fin and lies in the cover columns outside the base is false both ways. So 3 is removed from r8c2.
Practise it on a real puzzle. sudokUI generates a puzzle that needs Finned Swordfish 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 Swordfish, the puzzle's difficulty rating grows by 200. Its class is Unfair. It is needed in 1 in 68 puzzles that sudokUI generates. How the rating works.
More finned fish
Finned X-Wing · Sashimi X-Wing · Sashimi Swordfish · Finned Jellyfish · Sashimi Jellyfish