## Difficulty

## Description

The **Swordfish** is like an X-Wing, but instead of it consisting of 2x2 cells, it is consisting of **3x3 cells**. It is also called a **3x3 Fish**.

The **X-Wing** allows us to eliminate all those 5 candidates.

The **Swordfish** allows us to do the exact same thing. The only difference is, that this configuration is a bit harder to spot, because it consists of nine instead of four cells.

## Why does it work?

We have already shown why this elimination is correct for an X-Wing (by showing all two possible outcomes).

Now assume a cell of the Swordfish to be correct (**5**).

If you do that, there cannot be any **5**s in the same row and column.

What remains of the Swordfish is an X-Wing for which we have already shown that we can eliminate all candidates from the rows/columns.

This is true, regardless of what cell of the Swordfish we assume to be correct.

## Another way of showing it

We know for a Row-to-Column-Swordfish that we have to place a candidate in each of the three rows.

So there must be exactly one **5** in the first row of the Swordfish, one **5** in its second row, and exactly one in its third row.

Now if any of the cells were a **5**, ...

... we would eliminate one column of the swordfish completely.

Now there is no way of dividing three **5**s into two columns without breaking the rules of Sudoku.

This is of course also true for even bigger fish like the Jellyfish.

## Incomplete Swordfish

An incomplete Swordfish is **as useful as a complete Swordfish**.

It makes no difference if the candidate was ruled out from some of the Swordfish's cells.

**This** is still a Swordfish, and we can still eliminate the candidate from all of its rows or columns.

In theory, even something as simple as this could be seen as a Swordfish, but, of course, the **5**s in the rows are nothing more than Hidden Singles. That is why the sudoku.coach Solver would never find a Swordfish with fewer than two candidates in a row/column.