Swordfish
A Swordfish is an X-Wing scaled up from two lines to three. Find three rows in which a digit is confined to two or three cells each, such that all those cells fall within the same three columns. The digit must then be placed once in each of the three rows, and those placements necessarily occupy the three columns one apiece — so the digit can be removed from those columns everywhere outside the pattern.
An example: suppose the digit 1 can go only in columns 2 and 5 of row 1, columns 5 and 7 of row 4, and columns 2 and 7 of row 8. Across the three rows, every candidate falls in columns 2, 5 and 7. Three rows need three 1s; those 1s must land in the three columns without repeating a column. However the placements resolve, columns 2, 5 and 7 all get their 1 from rows 1, 4 or 8 — so 1 comes off any other cell in those columns, say r6c5 or r3c7.
Two points that trip solvers up. First, rows may hold only two candidates each, as in the example — the fish doesn't need a full three-by-three grid of nine cells, only that the union of columns is exactly three. Second, the pattern has the same row/column duality as the X-Wing: three columns confined to three shared rows eliminates along those rows instead.
Swordfish are rarer and harder to spot than X-Wings, so hunt them only after two-line patterns fail. A digit with many candidates spread thinly — two or three per line across several lines — is the promising terrain.
- List, per digit, the rows with two or three candidate cells; any three whose columns pool to exactly three form the fish.
- Rows with just two candidates are fine — the three-column union is the requirement, not nine filled corners.
- Remember duality: check the column-based orientation too, eliminating along rows.