The Swordfish is an advanced sudoku technique where a single candidate digit is confined to the same three columns across three rows (or vice versa), letting you eliminate that digit from every other cell in those lines. It is the three-row extension of the X-Wing pattern and mainly appears in hard and expert puzzles.
What Is the Swordfish Technique?
A Swordfish occurs when a specific candidate number appears in only two or three cells in each of exactly three rows, and all of those cells fall within exactly the same three columns. Because each of the three rows must place the digit somewhere inside those three columns, the digit cannot appear anywhere else in those columns — so you can safely delete it from all other cells in the three columns (sudokupulse.com).
The same logic works transposed: three columns whose candidates align within the same three rows let you eliminate along the rows instead. The technique is a single-digit solving method, documented in the classic reference Sudopedia as a basic fish pattern with 3 rows and 3 columns (sudopedia.enjoysudoku.com).
Swordfish vs X-Wing vs Jellyfish
The Swordfish belongs to a family of patterns collectively called fish:
| Pattern | Base sets | Cover sets | Eliminates from |
|---|---|---|---|
| X-Wing (2-fish) | 2 rows | 2 columns | Other cells in the 2 columns |
| Swordfish (3-fish) | 3 rows | 3 columns | Other cells in the 3 columns |
| Jellyfish (4-fish) | 4 rows | 4 columns | Other cells in the 4 columns |
If you have not learned X-Wing yet, start there — the Swordfish is the same logic on a bigger frame (livesudoku.com).
How the Swordfish Works: A Worked Example
Imagine the digit 6 is the "fish digit" and rows 1, 6 and 9 are the base rows. In each of those rows, the candidate 6 appears in only two or three cells — and every one of those cells sits in columns 2, 5 or 8.
Walk through the logic:
- Row 1 must place its 6 in column 2, 5 or 8.
- Row 6 must place its 6 in column 2, 5 or 8.
- Row 9 must place its 6 in column 2, 5 or 8.
- Since no column can hold two 6s, the three 6s occupy one cell in each of the three columns.
- Therefore no other cell in columns 2, 5 or 8 can contain a 6.
You now delete every other candidate 6 in those three columns. This often reveals hidden singles or naked pairs that unblock the whole puzzle (sudokustation.com). Want more grids to practise on? Our guide to sudoku puzzle books rounds up collections graded all the way up to expert level.
Do All Three Rows Need Three Candidates?
No. Each base row may contain two or three candidate cells — a minimum Swordfish has just six cells total, while a perfect one has nine. What matters is that every candidate falls inside the same three columns. A perfect 3×3, nine-cell pattern is actually quite rare; most real Swordfish are ragged-looking (livesudoku.com).
This flexibility is what makes the pattern both common enough to matter and hard to spot. Beginners often hunt for the textbook nine-cell square and miss the ragged six- or seven-cell versions hiding in real puzzles. When scanning, stop counting cells and start checking columns: find any three rows where the digit's candidates all squeeze into three shared columns, regardless of how lopsided the shape looks. The eliminations follow from the column confinement, not from the shape's symmetry — and the ugliest Swordfish cracks the puzzle just as cleanly as the prettiest.
How to Spot a Swordfish: Step by Step
- Finish the basics first. Swordfish is rare in easy and medium puzzles — only hunt for it when singles, locked candidates, pairs and X-Wing have run out (livesudoku.com).
- Pick one digit and highlight all its remaining candidates across the grid.
- Scan rows for three rows where the digit appears in only two or three cells each.
- Check alignment — do all those cells fall within the same three columns? If yes, you have a Swordfish.
- Eliminate the digit from every other cell in those three columns.
- Repeat with columns as base sets, eliminating along rows.
Full candidate notation (pencil marks in every empty cell) is essentially a prerequisite — without it, the pattern is invisible (sudokupulse.com).
Finned and Sashimi Swordfish
Not every near-miss is useless. In a finned Swordfish, extra candidates ("fins") appear in the same box as part of the pattern; if the fin cells share a box, eliminations can still be made in the cells that see all fins. A sashimi Swordfish is a finned pattern where one base set has candidates only in the fin cells — it still yields eliminations. These are genuinely advanced; master the basic Swordfish first (sudokubliss.com).
Think of fins as the exceptions that prove the pattern: the near-perfect Swordfish would work if one stray candidate weren't sitting outside the columns. Rather than discarding the whole pattern, advanced solvers check whether that stray "fin" sits in a box shared with part of the pattern — if so, the cells that can "see" every fin still lose the digit. Sashimi variants take this further: they look broken, with one row contributing candidates only through its fins, yet the elimination logic holds. You will rarely need these outside the very hardest expert grids, but recognising them means never throwing away a almost-Swordfish that could still crack the puzzle.
Swordfish Practice: Building Pattern Recognition
Reading about a Swordfish is one thing; spotting one in a live grid is another. The fastest way to build the pattern recognition is digit-by-digit drilling:
- Take a hard puzzle you have already pencil-marked. Do not start from scratch — use a grid where the basics are exhausted and the puzzle is genuinely stuck.
- Highlight one digit at a time. Most apps and websites let you highlight all candidates of a single digit. With everything else dimmed, row/column alignments pop out visually.
- Count rows, not cells. Scan for three rows where the highlighted digit appears in only two or three cells each. Ignore everything else.
- Check the columns. If those cells share exactly three columns, you have found it — eliminate along the columns and watch the grid unlock.
- Repeat for every digit. A full Swordfish hunt covers all nine digits, but with practice you will learn to prioritise digits with sparse candidates.
Do this on ten hard puzzles and the Swordfish stops being a diagram in an article and becomes something your eyes find on their own.
When to Use the Swordfish
- Difficulty level — almost exclusively hard, evil and expert puzzles. The Daily Telegraph's Tough sudoku regularly demands techniques at exactly this level.
- Before it — try X-Wing first; it is simpler and more common.
- After it — if Swordfish does not crack the puzzle, the next step up is Jellyfish, then forcing chains and trial-and-error techniques.
- Frequency — expect to use it rarely. Going weeks between Swordfish sightings is normal even for daily solvers.
What is the Swordfish technique in sudoku?
The Swordfish is an advanced elimination technique: when one digit’s candidates are confined to the same three columns across three rows (or the same three rows across three columns), you can remove that digit from all other cells in those lines.
What is the difference between X-Wing and Swordfish?
X-Wing uses two base rows and two columns; Swordfish uses three of each. Both restrict a single digit and eliminate it from the crossing lines — Swordfish is simply the larger version (livesudoku.com).
How rare is a Swordfish?
It is an advanced pattern that mostly appears in the hardest puzzles. Always check simpler techniques first; a Swordfish search is worthwhile when everything else is stuck (livesudoku.com).
Do I need pencil marks to find a Swordfish?
Yes, effectively. The pattern is defined by candidate positions, so you need full candidate notation in every empty cell before the Swordfish becomes visible (sudokupulse.com).
Can a Swordfish be found in columns instead of rows?
Yes. A column-based Swordfish uses three columns as base sets aligned within three rows, and eliminates the digit from other cells in those rows. The logic is identical (freepuzzles.net).
What comes after Swordfish?
The Jellyfish (4×4 fish pattern) is the direct next step, followed by advanced chaining techniques. In practice, most expert puzzles yield to Swordfish or simpler methods.
Can a Swordfish overlap with an X-Wing?
Yes. A Swordfish pattern can contain smaller X-Wing sub-patterns inside it — any two of the three rows that happen to align in two columns form an X-Wing on their own. Finding the X-Wing first is fine; the Swordfish simply gives you the eliminations from all three rows at once.
How do I know I have found a real Swordfish and not a coincidence?
Verify the definition strictly: exactly three base rows, each with two or three candidates, all inside the same three columns — no strays. If a fourth column sneaks in, it is not a Swordfish (though it might be a finned variant). When in doubt, do not eliminate.
Is Swordfish guessing?
No. It is a clean logical deduction, not trial and error. Every elimination follows necessarily from the constraint that each row, column and box holds each digit once (freepuzzles.net).