The idea
Take a digit and find three rows in which it can only go in cells belonging to the same three columns. Each row does not need all three columns — two is enough — but between them the three rows use only those three columns.
Those three rows must place the digit three times, one per row, and every one of those placements lands in one of the three columns. That fills all three columns, so no other cell in any of them can hold the digit.
Across three rows the digit is confined to the same three columns, so it can be removed from those columns elsewhere — here from the marked cell.
The same pattern, one size up
| Pattern | Lines | Digit appears in each line |
|---|---|---|
| X-Wing | 2 rows, 2 columns | Exactly twice |
| Swordfish | 3 rows, 3 columns | Two or three times |
| Jellyfish | 4 rows, 4 columns | Two to four times |
Finding one
Work with one digit at a time. Write down the columns in which it appears for each row, ignoring rows where it appears four times or more. Then look for three rows whose column sets fit inside a group of three. It is bookkeeping rather than intuition, which is why solvers usually check for XY-Wings first.
Is it worth learning?
For most solvers, only after X-Wings and XY-Wings are second nature — Swordfish is genuinely rare. But an expert grid does occasionally come down to one, and no other pattern will do the same work.
Common questions
What is a Swordfish in Sudoku?
A digit confined to the same three columns across three different rows. It can then be eliminated from those columns in every other row — or the same argument with rows and columns swapped.
How is it different from an X-Wing?
It is the same logic across three lines instead of two, and each line may hold the digit two or three times rather than exactly twice.
How common is a Swordfish?
Uncommon. Most expert puzzles are settled by an X-Wing, an XY-Wing or a chain of pairs long before a Swordfish is needed.