Swordfish

Volodymyr Sakhan  · 

Swordfish is an advanced sudoku technique that extends the logic of X-Wing from two rows to three. Like its predecessor, Swordfish works entirely with candidates — so you will need pencil marks before you can apply it.

When a digit can go in only two or three cells across each of three rows, and all those cells share the same three columns, the digit is locked into those rows for those columns. Every other occurrence of that digit in the three columns can be safely eliminated.

What Is Swordfish?

Swordfish is a fish pattern that spans three rows and three columns. Pick a candidate digit and find three rows where it appears in only two or three cells. If all those cells fall within exactly the same three columns, you have a Swordfish.

The key insight: wherever the digit lands in those three rows, it must occupy each of the three columns exactly once. That means no other cell in those three columns can hold the same digit — so any candidate for it outside the pattern cells can be eliminated.

When to Use Swordfish

Swordfish is an advanced technique. Reach for it after simpler methods have stalled:

Step-by-Step Example

The examples below show a row-based Swordfish and a column-based Swordfish. Both follow identical logic — only the scanning direction is swapped.

Row-Based Swordfish — Digit 1

In this puzzle, digit 1 forms a Swordfish across rows 2, 5, and 8 — all confined to columns A, D, and E.

  1. Fill in pencil marks. Focus on digit 1.
  2. In row 2, digit 1 can only go in A2 or D2 (two candidates in columns A and D).
  3. In row 5, digit 1 can only go in A5, D5, or E5 (three candidates in columns A, D, and E).
  4. In row 8, digit 1 can only go in D8 or E8 (two candidates in columns D and E).
  5. All seven cells fall within columns A, D, and E — exactly three columns.
  6. Column A has candidates in rows 2 and 5; column D in rows 2, 5, and 8; column E in rows 5 and 8. Every column is represented in at least two of the three rows.
  7. Wherever 1 lands in rows 2, 5, and 8, it fills columns A, D, and E once each. No other row in those columns can hold 1.
  8. Eliminate 1 from A1, D1, A3, D3, E3, D4, and E4 (shown in orange).
134569145827158715639241345621457179895167137842134618138257927895463172139548641681872939324715ABCDEFGHI123456789
Row-based Swordfish: digit 1 in rows 2, 5, 8 (green) confined to columns A, D, E. Orange cells lose candidate 1.

The 2-3-2 shape — row 5 has three candidates while rows 2 and 8 have two each — is the most common Swordfish form. It works just as well as the symmetric 2-2-2 pattern.

Column-Based Swordfish — Digit 5

Here, digit 5 forms a Swordfish across columns A, D, and I — all confined to rows 2, 5, and 6.

  1. Fill in pencil marks. Focus on digit 5.
  2. In column A, digit 5 can only go in A2 or A6 (two candidates in rows 2 and 6).
  3. In column D, digit 5 can only go in D2 or D5 (two candidates in rows 2 and 5).
  4. In column I, digit 5 can only go in I5 or I6 (two candidates in rows 5 and 6).
  5. All six cells fall within rows 2, 5, and 6 — exactly three rows.
  6. Row 2 has candidates in columns A and D; row 5 in D and I; row 6 in A and I. Every row is represented in at least two of the three columns.
  7. Wherever 5 lands in columns A, D, and I, it fills rows 2, 5, and 6 once each. No other column in those rows can hold 5.
  8. Eliminate 5 from F2, B5, F5, B6, and C6 (shown in orange).
69134825728594573164268597481279856792594567134515715795732864594165372827819438429651ABCDEFGHI123456789
Column-based Swordfish: digit 5 in columns A, D, I (green) confined to rows 2, 5, 6. Orange cells lose candidate 5.

The column-based Swordfish works identically to the row-based form. With rows and columns swapped, the clean 2-2-2 pattern is the simplest possible Swordfish shape.

Swordfish and Related Techniques

Swordfish belongs to the fish family of sudoku techniques, together with X-Wing (two rows/columns) and Jellyfish (four rows/columns). All three use the same principle: if a candidate is confined to N rows and N matching columns, it claims those columns entirely for those rows.

In practice, X-Wing is far more common than Swordfish, and Jellyfish is rarer still. When an X-Wing scan fails, try Swordfish before moving on to more complex strategies like Y-Wing. Fish patterns never overlap each other on the same digit — if X-Wing fires, there is no Swordfish on that digit in the same direction.

Practice Swordfish Online

Swordfish appears most often in hard and expert puzzles. The best way to internalize the pattern is to look for it during a real solve rather than in contrived examples.

Play a hard sudoku and try to spot the three-row pattern — or visit our full solving guide for a structured path through all sudoku techniques.

Frequently Asked Questions

Swordfish is a candidate-elimination technique. When a digit appears in only 2–3 cells in each of three rows, and all those cells share the same three columns, the digit must be placed in those rows for those columns — so the candidate can be eliminated from every other cell in those three columns.

Enable pencil marks, then pick a digit. Scan the rows for any three where the digit appears in only 2–3 cells. Check whether all those cells fall within the same three columns. If they do, and each column has candidates from at least two of the three rows, you have a Swordfish — eliminate the digit from all other cells in those three columns.

X-Wing uses two rows and two columns; Swordfish uses three rows and three columns. The logic is the same — a digit locked across matching rows and columns — but Swordfish catches more complex candidate distributions that X-Wing cannot handle.

Yes. A column-based Swordfish works identically: find three columns where a digit appears in only 2–3 cells each, all confined to the same three rows. The digit is then locked into those columns for those rows, and you can eliminate it from every other cell in those three rows.

Ready to practice Swordfish? Play a hard sudoku and look for the three-row pattern — or create a free account to track your progress.