The Swordfish Technique in Sudoku
Comfortable with the X-Wing? The Swordfish is its bigger sibling. It takes the same single-digit logic and stretches it from two rows and two columns to three. You will rarely need it, but on the hardest grids a Swordfish can be the one move that breaks a deadlock. Here is what it is, how to recognise it, and why the elimination is sound.
How to use the Swordfish, step by step
- Work from full pencil marks — like the X-Wing, the Swordfish lives in your candidate notes.
- Pick one digit and chase it across the whole grid, a single number at a time.
- Find three rows where that digit appears in only two or three cells each.
- Check the columns — if all of those candidates fall within the same three columns, you have a Swordfish.
- Eliminate the digit from those three columns everywhere outside the nine-cell frame.
- Re-scan for the singles the eliminations open up.
What is a Swordfish?
A Swordfish is a single-digit pattern spread across three rows and three columns: in each of three rows the digit is restricted to the same set of three columns. Because those nine intersection cells must hold the digit once per row and once per column, it can be removed from the rest of those three columns. It is the exact idea behind the X-Wing, simply scaled up by one.
How to spot a Swordfish
Chase one digit and look for three rows where it has only two or three possible cells, all sharing the same three columns. Unlike the X-Wing, the rows do not each need exactly two candidates — two or three is fine, as long as every candidate stays inside the three shared columns. As always it works the other way round too: three columns whose candidates line up on three rows.
- Full pencil marks are essential — a Swordfish is invisible without them.
- It is genuinely rare; only reach for it when singles, pairs and the X-Wing have stalled.
Swordfish, X-Wing and Jellyfish
These are the “fish” techniques, and they form a simple ladder by size: the X-Wing uses two lines, the Swordfish three, and the Jellyfish four. Each confines one digit to a grid of intersections and clears it from the rest of those lines. Master the X-Wing, then the Swordfish, and the larger fish need no new ideas — only a wider net. For the full toolkit see Sudoku tips & strategy.
Swordfish in Sudoku: FAQ
What is a Swordfish in Sudoku?
A Swordfish is a single-digit technique where a number is confined to the same three columns across three different rows. Those intersections account for the digit in each of the three columns, so it can be eliminated from the rest of them. It is the X-Wing extended from two lines to three.
How is a Swordfish different from an X-Wing?
An X-Wing uses two rows and two columns; a Swordfish uses three of each. In an X-Wing every row has the digit in exactly two cells, while in a Swordfish each row may have two or three candidate cells, as long as they all fall inside the three shared columns.
How do you spot a Swordfish?
Work from full pencil marks and chase one digit. Look for three rows where that digit can only go in two or three cells, and check whether all those cells sit within the same three columns. If they do, you have a Swordfish and can eliminate the digit from those columns elsewhere.
When do you need a Swordfish?
Almost never on Easy through Hard puzzles — singles, pairs, pointing pairs and the occasional X-Wing solve those. The Swordfish only earns its keep on Expert and Evil grids where every simpler technique has run out.