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

# Finned Swordfish

A **Finned Swordfish** is the finned version of a Swordfish. It is a single-digit solving technique.

The formal definition for a Finned Swordfish "in the rows" is:

- When 3 rows only have candidates for digit
**X** in 3 columns and a single box that intersects with at least one of these 3 columns, then we can eliminate all candidates for digit **X** inside that box, which are located on one of these 3 columns, but not on one of the 3 rows.

Swap the words "rows" and "columns" in this definition for a Finned Swordfish "in the columns".

## How it works

The following fish diagram illustrates how a Finned Swordfish "in the columns" causes eliminations.

Like the basic Swordfish, you do not need a candidate in each cell marked **X**. In fish techniques, the cells without a candidate are more important.

## Example

The following example shows a Finned Swordfish "in the columns" for the digit 7 is shown in yellow in the grid, with a fin located at **r6c7**.

This Finned Swordfish leads to the elimination of 7 from **r4c9**, which is shown in blue. For clarity, the fish diagram for this example is shown below.

X . . | . . X | X . .
/ . . | . . / | / . .
/ . . | . . / | / . .
------+-------+------
X . . | . . X | X * *
X . . | . . X | X * *
/ . . | . . / | F . .
------+-------+------
/ . . | . . / | / . .
/ . . | . . / | / . .
/ . . | . . / | / . .

An argument for the elimination of 7 from **r4c9** is by cases on the fin at **r6c7**. __Case 1__ - the fin has value 7: then nothing else in box 6 can have value 7. __Case 2__ - the fin has value <>7: then the remaining yellow cells form a swordfish-in-columns and so nothing else in rows 1, 4 or 5 can have value 7. __Result__: common to both of these cases are the deductions that none of r4c8, **r4c9**, r5c8 or r5c9 can have value 7.

## See also

This page was last modified 09:26, 1 January 2007.