Swordfish is similar to X-Wing, but instead of using a rectangle of four squares, it uses a grid of up to nine squares. You can use the swordfish strategy to eliminate candidates.

When to Use Swordfish

You can use Swordfish when there are three rows that each contain either two or three of a given candidate, and those numbers are aligned on exactly three columns (as in the above example, with the nine squares highlighted in green). Alternatively, you can use swordfish when there are three columns that each contain either two or three of a given candidate, and those numbers are aligned on exactly three rows.

These numbers form a grid of nine squares. At least six of these nine squares must be unsolved and contain the same candidate. For it to be a valid swordfish, either the rows of the swordfish grid contain at least two and no more than three of this candidate, with at least one of the columns containing more than three of this candidate, OR, the columns of the swordfish grid contain at least two and no more than three of this candidate, with at least one of the rows containing more than three of this candidate.

In the example above, the rows of the green squares contain at least two and no more than three 8s, but two of the columns contain other 8s.

How to Use Swordfish

In a swordfish where the rows contain no more than three of a candidate, the columns can contain more than three. When this is the case (as in the example image), you can eliminate any candidates on the columns except for the ones that form the swordfish. In this case, there are 8s in the red squares. The 8s are highlighted yellow. These yellow 8s can be eliminated.

In a swordfish where the columns contain no more than three of a candidate, the rows can contain more than three. When this is the case, you can eliminate any candidates on the rows except for the ones that form the swordfish.

Interactive Example

Some 2s can be removed from the following interactive puzzle using the Swordfish strategy.