Swordfish
An X-Wing scaled to three rows and three columns. A candidate confined to three shared columns across three rows is eliminated elsewhere in those columns.
Candidate 7 in three rows is confined to columns 2, 5, and 8 between them.
How to spot it
- 1Find three rows where a candidate appears in two or three cells.
- 2Verify all those cells fall within the same three columns.
- 3Eliminate the candidate from the rest of those columns.
When to use
On expert puzzles when X-Wings and chains don't crack it.
When not to
If a shorter chain achieves the same elimination, prefer it — swordfish are error-prone by hand.
Common mistake
Requiring exactly three cells per row. Two or three each is fine, as long as the columns align.
Worked Example
A swordfish is an X-Wing stretched to three rows and three columns. Suppose candidate 7 in three rows is confined to cells that, between them, only ever use three columns — not necessarily three cells in every row, just two or three, as long as every one of them falls in the same three columns.
Between the three rows, those three columns must supply their 7 from within this pattern. That means no other cell in those three columns — outside the three rows — can be a 7, so every stray 7 candidate there can be eliminated. It's a technique reserved for expert puzzles once X-Wings and chains alone don't crack it.