Swordfish in Sudoku: How to Spot It
Learn the Swordfish Sudoku technique: three rows and three columns lock a candidate into a fish pattern.
Guided example
See the Swordfish on this board
Read this as one teaching unit: first identify the active unit, then the marked pattern cells, then the candidates removed or maintained.
Quick answer
A Swordfish in Sudoku is a fish pattern where one candidate appears only in the same three columns across three rows, or the same three rows across three columns. You can remove that candidate from the rest of those columns or rows.
How to spot it
Follow the board in order
- 1 Pick one candidate number and scan rows or columns only for that number.
- 2 Find three rows where the candidate appears in no more than three matching columns.
- 3 Confirm each selected row has its candidate positions only inside those three columns; do not include a row with an extra candidate outside the fish.
- 4 Remove the candidate from other cells in those columns.
What is a Swordfish?
A Swordfish is an expert fish pattern. It tracks one candidate across three rows and three columns.
If that candidate is locked into the same three columns across three rows, then the candidate can be removed from the rest of those columns.
When to look for it
Look for Swordfish only after simpler scans fail. It is most useful in harder puzzles where candidate notes are already well maintained.
Start from X-Wing logic. If two rows are not enough, check whether a third row completes the same column pattern.
Example walkthrough
This walkthrough uses a real expert app puzzle tagged Swordfish. Candidate 6 is confined to the blue cells across three rows and three columns. Because those three columns must take their 6s from the fish rows, the green cells outside the fish lose candidate 6.
Why the elimination works
The fish rows and columns form a locked structure. If a 7 were placed outside the fish rows but inside a fish column, one of the fish rows would lose its necessary place for 7.
So the candidate is eliminated from non-fish cells in the fish columns.
Swordfish vs. X-Wing
An X-Wing uses two rows and two columns.
A Swordfish uses three rows and three columns. The logic is similar, but the scan is wider and easier to misread.
Common mistake
Do not mix candidates. A Swordfish for 7 must track only 7s. If one corner is actually a different candidate, the pattern is invalid.
FAQ
What is a Swordfish in Sudoku?
A Swordfish is a three-row/three-column fish pattern for one candidate number.
Is Swordfish harder than X-Wing?
Yes. Swordfish is the three-line version of the same fish logic used by X-Wing.
Which candidate do I track in Swordfish?
Track one candidate at a time. Do not mix different numbers inside the same fish pattern.
Do beginners need Swordfish?
Usually no. Master singles, pairs, triples, and X-Wing before relying on Swordfish.
In this lesson
- Board example: see the pattern first.
- Walkthrough: connect each highlight to the rule.
- FAQ: check edge cases after the move is clear.
Practice this technique in Sudoku Coach
Read the pattern, then practice it step by step with guided hints that explain why the move works.
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Naked Triple in Sudoku: How to Spot It
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XY-Wing in Sudoku: How to Spot It
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