Finned Swordfish

Level 9 · Purple Belt

Strategy:

Finned & Sashimi Fish

Requires:

Finned X-Wing Swordfish

Learn after:

Finned Jellyfish

Overview

A Finned Swordfish is a Swordfish pattern with extra candidates forming a fin. Eliminations are limited to cells seeing both the fish and the fin.

How It Works

Find a Swordfish where one row has candidates outside the three columns.
The extra candidates are the fin.
Eliminate only from cells that see both a cover column and the fin's block.

Example

Starting position

Look for Finned Swordfish.

Sudoku grid showing the initial puzzle state for Look for Finned Swordfish.

1

Rows 1, 4 and 6 form a Swordfish on 1

Base rows 1, 4 and 6 form a Swordfish for digit 1 in cover columns 1, 2 and 4.

Sudoku grid illustrating step 1: Rows 1, 4 and 6 form a Swordfish on 1 — Base rows 1, 4 and 6 form a Swordfish for digit 1 in cover columns 1, 2 and 4.

2

r1c5 is already filled

r1c5 is already filled with 6.

Sudoku grid illustrating step 2: r1c5 is already filled — r1c5 is already filled with 6.

3

r1c6 is already filled

r1c6 is already filled with 3.

Sudoku grid illustrating step 3: r1c6 is already filled — r1c6 is already filled with 3.

4

r1c7 is already filled

r1c7 is already filled with 2.

Sudoku grid illustrating step 4: r1c7 is already filled — r1c7 is already filled with 2.

5

r1c8 is already filled

r1c8 is already filled with 7.

Sudoku grid illustrating step 5: r1c8 is already filled — r1c8 is already filled with 7.

6

1 is blocked from r1c9 by the column

Digit 1 cannot go in r1c9 because r5c9 is already in this column. Remove the entry or choose a different digit.

Sudoku grid illustrating step 6: 1 is blocked from r1c9 by the column — Digit 1 cannot go in r1c9 because r5c9 is already in this column. Remove the entry or choose a different digit.

7

r4c3 is already filled

r4c3 is already filled with 9.

Sudoku grid illustrating step 7: r4c3 is already filled — r4c3 is already filled with 9.

8

1 is blocked from r4c5 by the column

Digit 1 cannot go in r4c5 because r2c5 is already in this column. Remove the entry or choose a different digit.

Sudoku grid illustrating step 8: 1 is blocked from r4c5 by the column — Digit 1 cannot go in r4c5 because r2c5 is already in this column. Remove the entry or choose a different digit.

9

1 is blocked from r4c6 by the column

Digit 1 cannot go in r4c6 because r7c6 is already in this column. Remove the entry or choose a different digit.

Sudoku grid illustrating step 9: 1 is blocked from r4c6 by the column — Digit 1 cannot go in r4c6 because r7c6 is already in this column. Remove the entry or choose a different digit.

10

r4c7 is already filled

r4c7 is already filled with 7.

Sudoku grid illustrating step 10: r4c7 is already filled — r4c7 is already filled with 7.

11

r4c8 is already filled

r4c8 is already filled with 2.

Sudoku grid illustrating step 11: r4c8 is already filled — r4c8 is already filled with 2.

12

1 is blocked from r4c9 by the column

Digit 1 cannot go in r4c9 because r5c9 is already in this column. Remove the entry or choose a different digit.

Sudoku grid illustrating step 12: 1 is blocked from r4c9 by the column — Digit 1 cannot go in r4c9 because r5c9 is already in this column. Remove the entry or choose a different digit.

13

r6c3 is already filled

r6c3 is already filled with 7.

Sudoku grid illustrating step 13: r6c3 is already filled — r6c3 is already filled with 7.

14

1 is blocked from r6c5 by the column

Digit 1 cannot go in r6c5 because r2c5 is already in this column. Remove the entry or choose a different digit.

Sudoku grid illustrating step 14: 1 is blocked from r6c5 by the column — Digit 1 cannot go in r6c5 because r2c5 is already in this column. Remove the entry or choose a different digit.

15

r6c6 is already filled

r6c6 is already filled with 2.

Sudoku grid illustrating step 15: r6c6 is already filled — r6c6 is already filled with 2.

16

r6c7 is already filled

r6c7 is already filled with 9.

Sudoku grid illustrating step 16: r6c7 is already filled — r6c7 is already filled with 9.

17

1 is blocked from r6c8 by the box

Digit 1 cannot go in r6c8 because r5c9 is already in this box. Remove the entry or choose a different digit.

Sudoku grid illustrating step 17: 1 is blocked from r6c8 by the box — Digit 1 cannot go in r6c8 because r5c9 is already in this box. Remove the entry or choose a different digit.

18

1 is blocked from r6c9 by the column

Digit 1 cannot go in r6c9 because r5c9 is already in this column. Remove the entry or choose a different digit.

Sudoku grid illustrating step 18: 1 is blocked from r6c9 by the column — Digit 1 cannot go in r6c9 because r5c9 is already in this column. Remove the entry or choose a different digit.

19

The fin sits in box 1

One base row has extra candidates in box 1. That box is the fin, making this a Finned Swordfish.

Sudoku grid illustrating step 19: The fin sits in box 1 — One base row has extra candidates in box 1. That box is the fin, making this a Finned Swordfish.

20

Only box 1 can lose 1

Because of the fin, only cells in box 1 that also lie in columns 1, 2 and 4 can lose digit 1.

Sudoku grid illustrating step 20: Only box 1 can lose 1 — Because of the fin, only cells in box 1 that also lie in columns 1, 2 and 4 can lose digit 1.

21

Cells that see both fish and fin

These cells see both the fish structure and the fin, so they are removal targets: r3c1 and r3c2.

Sudoku grid illustrating step 21: Cells that see both fish and fin — These cells see both the fish structure and the fin, so they are removal targets: r3c1 and r3c2.

22

Remove 1 from the fin-affected cells

Remove 1 from these fin-affected cells: r3c1 and r3c2.

Sudoku grid illustrating step 22: Remove 1 from the fin-affected cells — Remove 1 from these fin-affected cells: r3c1 and r3c2.

23

Result

Result after applying the technique.

Sudoku grid showing the result after applying the technique

Tips & Patterns

Finned Swordfish are rare but appear in hard puzzles.
The fin must share a block with one of the proper Swordfish cells.