Difficulty tier T5
Swordfish Sudoku Technique: The X-Wing, Scaled Up
The swordfish looks fearsome and is really just an X-Wing that grew. If you understand the X-Wing, you already understand the logic — a swordfish only adds a third line.
What a swordfish is
Take a single candidate digit. A swordfish forms when that digit is confined to at most three cells in each of three rows, and all of those cells fall within only three columns. Because the digit must appear once in each of those three rows, and only three columns are available to hold it, those three columns will absorb all three copies.
So the digit can be eliminated from every other cell in those three columns, outside the pattern. As with the X-Wing, the pattern also runs the other way — three columns confined to three rows, eliminating across the rows.
The cells don't have to fill a neat 3×3 grid — each row can use two or three of the three columns. What matters is that the whole thing stays inside three rows and three columns.
Same logic as the X-Wing, one line bigger
| Pattern | Lines used | Eliminates from |
|---|---|---|
| X-Wing | 2 rows × 2 columns | The 2 crossing lines |
| Swordfish | 3 rows × 3 columns | The 3 crossing lines |
That's the entire difference. Both belong to the "fish" family — a candidate locked into N lines can be cleared from the N crossing lines.
The ceiling: swordfish is tier T5 in Speedoku's difficulty model — the hardest technique the game ever requires, alongside the unique rectangle. Beyond it lie chains and colouring, which we leave out on purpose: they fight the speed identity.
When it's worth hunting for
Rarely. A swordfish is hard to spot and appears only in expert puzzles, so on everyday boards the time is better spent making the cheap moves instant. Treat "this puzzle needs a swordfish" mainly as a signal that you've hit a genuinely hard grid — see what makes a sudoku hard. Learn it so nothing can stop you; don't expect to use it often.
The pattern-spotting only comes with reps on hard boards: practice free or take today's Daily Challenge.
Frequently asked questions
What is a swordfish in sudoku?
- A swordfish is a single-digit elimination pattern: a candidate is confined to at most three cells in each of three rows, and those cells fall within only three columns. Because the digit must occupy one cell per row and only those three columns are available, it can be removed from every other cell in those three columns. It also works with rows and columns swapped.
How is a swordfish different from an X-Wing?
- It's the same idea scaled up. An X-Wing uses two rows and two columns; a swordfish uses three rows and three columns. The logic is identical — a candidate locked into a small set of lines can be eliminated from the crossing lines — just larger and harder to spot.
Do you need swordfish to solve most sudoku?
- No. Swordfish only appears in genuinely hard puzzles and is rare even there. The vast majority of puzzles are solved with singles, pairs, and pointing candidates. Learn swordfish for completeness so an expert grid can't stop you — not to get faster.