Swordfish
The X-Wing, one size up: three rows, three columns, one candidate pinned across the grid.
The pattern
Pick one digit. If you can find three rows where that digit's candidates all fall inside the same three columns, the digit is trapped: three rows need three copies, the three columns can absorb exactly one each. Every other cell in those columns loses the candidate. Swap rows and columns and the same logic runs the other way.
Across three rows, candidate 4 appears only in the same three columns. Three rows place three 4s — one per column — so 4 is removed from every other cell of those columns.
Why it works
It's pigeonhole logic. Each of the three rows must contain the digit somewhere, and by assumption each row's only options sit in the three shared columns. Three digits, three columns, and no column can hold two (they'd collide) — so each column takes exactly one, from one of those rows. Any other cell in a shared column would be a second copy. Gone.
The X-Wing is this same argument with two lines; a Swordfish is three. (Four lines makes a Jellyfish — same proof, rarer sighting.)
How to spot one
- Work one digit at a time. Swordfish hunting is digit-first, not cell-first — pick a digit with sparse candidates.
- List rows with 2–3 spots. Rows where the digit has only two or three candidate cells are your raw material.
- Look for a shared column set. Three such rows whose candidate columns fit inside one set of three columns — that's the fish.
- Don't demand all nine cells. The 2-2-2 cycle is legal and common; missing corners don't break the logic.
- Scan both orientations. Column-based Swordfish eliminate along rows — check both before moving on.
A puzzle that needs one
Generated and verified by our grader: singles and two naked pairs carry this puzzle a long way — then it takes a Swordfish to break through, and an XY-Wing to finish. That pair of moves earns it the Diabolical grade.
| 7 | 4 | 6 | ||||||
| 7 | 5 | 2 | 8 | |||||
| 6 | 8 | 3 | ||||||
| 6 | 8 | 1 | ||||||
| 2 | 4 | 1 | 6 | |||||
| 7 | 2 | |||||||
| 5 | 9 | |||||||
| 8 | 6 | 1 | 2 | 5 | ||||
| 8 |
Copy it into the difficulty grader to see the certificate yourself — then go fishing on a fresh board.
Common questions
What is the Swordfish technique in sudoku?
A Swordfish is the three-line version of the X-Wing: one candidate digit is confined, across three rows, to the same three columns (or across three columns to the same three rows). Those three lines must place three copies of the digit — one per crossing line — so the candidate is removed from every other cell of the crossing lines.
Do all nine Swordfish cells need the candidate?
No. Each of the three rows only needs the candidate in two or three of the shared columns — the 2-2-2 form is a perfectly good Swordfish, and it's the shape people miss. What matters is that none of the three rows offers the digit outside the three columns.
How hard is a Swordfish?
Stillgrid grades Swordfish in the Diabolical tier, alongside XY-Wing — harder than an X-Wing, easier than full chains. The certified sample on this page requires one Swordfish and one XY-Wing to finish.
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