Swordfish Technique

The Swordfish Technique extends the concept of the X-Wing pattern, identifying a scenario where a specific candidate appears across exactly three rows in the same three columns, or across exactly three columns in the same three rows. This advanced technique helps eliminate candidates from other cells within those rows or columns.

  • Row-based Swordfish: If a candidate appears in exactly three matching columns across three different rows, it can be eliminated from all other cells in those columns.
  • Column-based Swordfish: If a candidate appears in exactly three matching rows across three different columns, it can be eliminated from all other cells in those rows.
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In this puzzle, a Row-based Swordfish occurs where the number 1 appears in exactly three columns (columns 1, 4, and 7) in rows 1 and 3. As a result, the number 1 can be eliminated from all other cells in columns 1, 4, and 7.

Play this exact puzzle.

This puzzle combines Hidden Pairs and Singles along with the Swordfish technique for a truly rewarding solve.

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How to spot it

Find a digit confined to the same three columns across three rows (each row using two or three of those columns). The nine-cell lattice is the Swordfish.

When to use it

Try it after X-Wing fails. It is the same fish logic one size larger and clears stubborn candidates in hard puzzles.

Common mistakes

  • Requiring each row to use all three columns; a row may use only two of the three, as long as no row strays outside the three columns.
  • Eliminating along rows instead of columns for a row-based Swordfish.

Frequently asked questions

What is a Swordfish in Sudoku?
A Swordfish is a pattern where a candidate digit, across three rows, is confined to the same three columns. The digit can then be eliminated from every other cell in those three columns. The row and column roles can be swapped.
How is a Swordfish different from an X-Wing?
An X-Wing spans two rows and two columns; a Swordfish spans three rows and three columns. Both are fish patterns that eliminate a candidate from the crossing lines.
Do all three rows of a Swordfish need the digit in all three columns?
No. Each of the three rows must keep the digit within the three shared columns, but a row may use only two of them. What matters is that no row places the digit outside the three columns.
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