Swordfish Sudoku Strategy: Rows, Columns & Eliminations
A Swordfish extends the X-Wing idea from two rows and columns to three. For one candidate digit, choose three rows in which that digit appears only within the same three columns. At least one candidate in each row must be true, so every occurrence of that digit outside the three selected rows can be removed from those columns.
Three rows, three columns, one digit: the fish matters only when every candidate stays inside that frame.
What is a Swordfish?
A row-based Swordfish uses three base rows and three cover columns. In each base row, the target digit must have two or three candidates, all located in the cover columns. The pattern does not require all nine intersections to contain the digit. A column-based Swordfish is the same rule with rows and columns reversed.
How to find a Swordfish
- Choose one candidate digit. Scan its notes across the entire grid rather than mixing several digits.
- Find three restricted rows. Each selected row must place the digit only in the same set of three columns.
- Confirm every base row participates. Each row needs at least two candidates and no candidate outside the three cover columns.
- Eliminate from the cover columns. Remove the digit from cells in those columns that are outside the three base rows.
Example: a row-based Swordfish on 7
Suppose candidate 7 in rows 2, 5, and 8 appears only in columns 1, 4, and 9. Those three rows must place their 7s somewhere in those three columns.
- Mark the frame. Highlight rows 2, 5, 8 and columns 1, 4, 9.
- Check for leaks. If any selected row has another 7 outside the frame, the Swordfish is invalid.
- Make the eliminations. Delete candidate 7 from all other rows in columns 1, 4, and 9.
Common Swordfish mistakes
Forcing an exact shape
Candidates do not need to occupy all nine intersections or form a symmetrical picture.
Ignoring an outside candidate
One extra target candidate outside the cover columns breaks that base row.
Eliminating inside the fish
The valid removals are outside the base rows but inside the cover columns.
Swordfish practice checklist
- Finish singles and locked candidates before searching for a fish.
- Scan one digit at a time and compare rows with only two or three positions.
- State the three base rows and three cover columns before deleting anything.
Treat Swordfish as a constraint, not a picture. Name the target digit, verify every base row, then limit eliminations to the cover columns. That short verbal check prevents most false fish.
Swordfish FAQ
Is Swordfish harder than X-Wing?
Usually. It uses the same logic but tracks three base houses and three cover houses instead of two.
Can a Swordfish have three candidates in one row?
Yes, provided every candidate in each base row stays within the same three cover columns.
Does Swordfish place a digit directly?
Usually it removes candidates. Those eliminations may then create a single elsewhere.
