Mirrored from **Sudopedia**, the Free Sudoku Reference Guide

A **Swordfish** is a basic fish pattern with 3 rows and 3 columns. It is a single-digit solving technique.

## Contents |

- 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.

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.

- 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 3 lines of a Swordfish crossing in a single box. In that case, the Swordfish would degenerate into a series of Locked Candidates.

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.

As already pointed out, it may be difficult to spot a Swordfish in a pencilmark grid. But the analysis and discovery of basic fish (X-Wing, Swordfish, Jellyfish, Squirmbag) can be made easier by mapping the fish problem onto a Locked Set problem. See Fish and Subsets to find out how to perform this mapping.

The following puzzles will help you practice this technique:

000308002000040700001970080905003006037000520800500903070096100006030000400807000 000000000008206900109804206004602500700000008001307400603405709005903800000000000 800060000000007600025900010008604000400000006000108300010006790002400000000090004 003105400000070000604903705501000302070030050802000907107306204000010000006207500 260907043730000089000000000100306005000070000400502001000000000340000097920403016

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

- Fish
- X-Wing
- Jellyfish
- Finned Swordfish
- Sashimi Swordfish
- Franken Swordfish
- Mutant Swordfish
- Fish and Subsets