Swordfish
A Sudoku solving technique · level 5 · first needed around SE grade 7
By The Hardest Sudoku Team · Last updated
TL;DR
A Swordfish is the three-line generalization of the X-Wing: a digit confined to the same three columns across three rows, enabling column eliminations.
How does the Swordfish work?
Across three rows a digit appears only within the same three columns (two or three cells per row). The digit must fill three of those nine intersections, one per row and column — so it can be eliminated from those three columns in all other rows.
When does it apply?
When three rows (or columns) collectively restrict a digit to three shared columns (or rows).
A worked example
If 9 is confined to columns 1, 4 and 8 across rows 2, 5 and 9, then 9 is removed from those columns in every other row.
See where this fits on the difficulty scale on the SE grade page, or browse every technique in the glossary.