Swordfish tracks one digit across three rows or three columns. It is less visually tidy than X-Wing because a base unit can use two or three of the selected cover positions, but the candidate must remain confined to the same three lines.
What is a Swordfish in Sudoku?
A Swordfish occurs when all candidates for one digit in three rows are confined to the same three columns, or vice versa. Those three rows must place the digit once each within the three selected columns, so the digit can be removed from other cells in those columns outside the Swordfish rows.
What does a Swordfish example look like?
Suppose every candidate 4 in rows 1, 5 and 8 lies within columns 2, 6 and 9. Row 1 may use columns 2 and 9, row 5 may use columns 2, 6 and 9, and row 8 may use columns 6 and 9. The rows do not need identical pairs.
Each selected row needs one 4. Because all three placements are confined to three columns, those columns will receive their 4s from the selected rows. Remove candidate 4 from columns 2, 6 and 9 in every other row.
How do you verify a Swordfish?
- Track one digit only.
- Select three base rows containing candidates in no more than three columns.
- Confirm their combined cover-column set contains exactly three columns.
- Verify that every candidate for the digit in each base row is inside those covers.
- Remove the digit from the cover columns outside the base rows.
A base row may contain two or three candidates. Across the three bases, every cover column must participate in the pattern; otherwise you may have a smaller fish or no useful elimination.
How is Swordfish different from X-Wing?
X-Wing uses two base units and two cover units. Swordfish uses three of each. The same conservation argument applies: the selected bases need one placement each, and the same number of cover units contains every possible position.
Because Swordfish has more candidate arrangements, it is harder to see and easier to misidentify. Search for X-Wings first; a smaller valid fish is usually clearer and may simplify the grid before Swordfish is needed.
How should you practice Swordfish?
Start with candidate maps that highlight one digit. Practice naming the base and cover units before making an elimination. Say the proof aloud: “These three rows place 4 only in these three columns; therefore other 4s in the columns are impossible.”
After every removal, update candidates and scan for singles, locked candidates and subsets. Swordfish is an elimination tool, and its real payoff is often a simpler deduction created downstream.
Free tool / Paper-ready
Want to test the rule on your own puzzle?
Take or upload a clear Sudoku photo, review every recognized given, and choose one hint or the full solution. The editable transcription matters: correct any faint or misread digit before asking the solver to reason from the grid.
Open the Sudoku Solver from Photo →About our game: Detective Sudoku applies one-per-row-and-column deduction to suspects, rooms and truthful clues. It is not classic Sudoku, and its real hints explain a clue-driven deduction instead of giving away the answer or solution.
Frequently asked questions about Swordfish Sudoku
Must each Swordfish row have exactly two candidates?
No. A base row may have two or three, provided all candidates remain inside the same three cover columns.
Can Swordfish be column-based?
Yes. Choose three base columns whose candidates are confined to the same three rows.
Is Swordfish more advanced than X-Wing?
Yes. It applies the same fish logic to three base and cover units, creating more possible layouts.
Does Swordfish solve a cell?
It removes candidates. Those eliminations may expose a single or another simpler pattern.