Swordfish

Swordfish in the Rows

When all candidates for digit X in 3 rows are located in only 3 columns, we can eliminate all candidates for digit X from those 3 columns except those located on the defining 3 rows.

Here is a diagram that shows a Swordfish in the rows:

The candidates in the 3 defining rows are marked X. These candidates only occupy 3 columns. Because it is impossible to have more than one instance of a digit in a column, each instance of X in the defining rows must be placed in one of these 3 columns. In other words: the candidates for digit X in these 3 columns must be located in these 3 rows. This allows us to eliminate the remaining candidates in these 3 columns.

Each defining row can contain 2 or 3 candidates for digit X. A minimum Swordfish has a different appearance, but it still fits the definition:

A minimum Swordfish can also be seen as a Fishy Cycle.

Example

The diagrams are useful to understand how this technique works, but as you will see in the following example, a Swordfish is not easy to spot in a pencilmark grid.

Swordfish for digit 9 in rows 3,7,8 and columns 1,5,8.

Swordfish in the Columns

When all candidates for digit X in 3 columns are located in only 3 rows, we can eliminate all candidates for digit X from those 3 rows except those located on the defining 3 columns.

Here is the diagram for this version:

In this diagram, you can also see that is is possible to have 2 defining columns in a single chute. Two of the secondary lines can also be located in a single chute, but it is not possible to have 4 lines of a Swordfish crossing in a single box. In that case, the Swordfish would degenerate into a series of Locked Candidates.

Example

Here is an example of a Swordfish in the columns:

Minimum Swordfish for digit 3 in columns 1,5,7 and rows 1,5,7.

Practice Sudokus

The following puzzles will help you practice this technique:

Select and copy each string and paste it into a computer program that accepts this input.

Sudoku Games

• Sudoku 9981
• Samurai Sudoku
• Sudoku Printer
• Word Sudoku to Learn English
  Word Sudoku to Learn Chinese
  Word Sudoku to Learn French
  Word Sudoku to Learn German
  Word Sudoku to Learn Spanish

Free Sudoku Script for Your Website

Sudoku strategies

• Coloring
• Singles
• Fish
  • X-Wing
  • Swordfish
  • Squirmbag
  • Finned Fish
  • Sashimi Fish
  • Franken Fish
  • Mutant Fish
  • Kraken Fish
• Chains and Loops
• Intersections
• Subsets
• Wings
• Single Digit Patterns
• Uniqueness
• Almost Locked Sets

Printable Sudoku

• Kids Sudoku Puzzles
• Easy Sudoku Puzzles
• Standard Sudoko Puzzles
• Hard Sudoku Puzzles
• Expert Soduko Puzzles
• Extreme Soduku Puzzles
• Blank Sudoku Grids