Swordfish sudoku technique

A swordfish extends the X-Wing from two lines to three. Take a digit confined, in each of three rows, to cells that all fall within the same three columns — two or three cells per row both count. Those three columns must then supply the digit to the three rows, which uses up the columns entirely: any other candidate for the digit in those columns can be removed.

How to find a swordfish

1. Choose a digit and note which cells can take it in every row.
2. Search for three rows whose candidates for the digit fit inside a shared set of three columns.
3. Erase the digit from those three columns outside the pattern cells.
4. No luck row-wise? Transpose the search and look across columns instead.

The pattern on the grid

Worked example

Digit 6 appears only in r1c2+r1c5 of row 1, r4c2+r4c8 of row 4, and r8c5+r8c8 of row 8. Three rows, and the candidate cells fall inside columns 2, 5 and 8 — a swordfish. Those three columns must deliver the 6 to rows 1, 4 and 8, which exhausts them: 6s pencilled anywhere else in columns 2, 5 or 8 (say r6c2, r3c5 or r9c8) are gone.

When it fails

Rows with two candidate cells are the reliable skeleton. A row with four candidates can never be part of the fish, and a 'swordfish' whose columns number four is no swordfish at all — re-count before erasing.

Related techniques

• X-Wing
• Forcing Chain
• XY-Wing

All solving techniques