Sudoku techniques · Expert
Swordfish in Sudoku
A Swordfish is the three-line version of an X-Wing. A number's candidates in three rows all fall within the same three columns, so those three columns must absorb the number three times, once from each row, and every other cell in them loses it.
Also known as: 3-Fish.
Why it works
When a number's candidates in three rows all fall within the same three columns (or in three columns all fall within the same three rows), that candidate is removed everywhere else in those three crossing lines.
How to spot it
The trap is expecting each row to contribute exactly two cells. It does not have to.
- Each of the three base rows may hold two or three candidates for the number, as long as all of them fall inside the same three columns.
- Map the number's positions row by row and write down the column numbers; then look for three rows whose column sets union to three.
- Check the column-based version too, with rows and columns swapped.
Builds on
Worked example
These are the app's own lesson boards. Step through them the same way a hint does.
- 1
In each base row, 5 is a candidate in 2 or 3 cells. Taking the three base rows together, every 5 candidate sits in one of only three columns. This is a Swordfish!
- 2
The Swordfish means we can remove the candidate 5 from the three columns that hold it, in every cell outside the three base rows.