Finned Swordfish

Level 8 · Blue Belt

Strategy:

Finned & Sashimi Fish

Requires:

Finned X-Wing Swordfish

Learn after:

Finned Jellyfish Sashimi Swordfish

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

Look for Finned Swordfish.

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

1

Finned Swordfish

Check digit 6 in columns 1, 6 and 7. These columns form a Swordfish pattern in rows 2, 4 and 6.

Sudoku grid illustrating step 1: Finned Swordfish — Check digit 6 in columns 1, 6 and 7. These columns form a Swordfish pattern in rows 2, 4 and 6.

2

Finned Swordfish

r1c1 is already filled with 7.

Sudoku grid illustrating step 2: Finned Swordfish — r1c1 is already filled with 7.

3

Finned Swordfish

r3c1 is already filled with 4.

Sudoku grid illustrating step 3: Finned Swordfish — r3c1 is already filled with 4.

4

Finned Swordfish

r7c1 is already filled with 3.

Sudoku grid illustrating step 4: Finned Swordfish — r7c1 is already filled with 3.

5

Finned Swordfish

r8c1 is already filled with 2.

Sudoku grid illustrating step 5: Finned Swordfish — r8c1 is already filled with 2.

6

Finned Swordfish

r9c1 is already filled with 1.

Sudoku grid illustrating step 6: Finned Swordfish — r9c1 is already filled with 1.

7

Finned Swordfish

Digit 6 cannot go in r1c6 because r1c5 is already in this row. Remove the entry or choose a different digit.

Sudoku grid illustrating step 7: Finned Swordfish — Digit 6 cannot go in r1c6 because r1c5 is already in this row. Remove the entry or choose a different digit.

8

Finned Swordfish

Digit 6 cannot go in r3c6 because r1c5 is already in this box. Remove the entry or choose a different digit.

Sudoku grid illustrating step 8: Finned Swordfish — Digit 6 cannot go in r3c6 because r1c5 is already in this box. Remove the entry or choose a different digit.

9

Finned Swordfish

r5c6 is already filled with 1.

Sudoku grid illustrating step 9: Finned Swordfish — r5c6 is already filled with 1.

10

Finned Swordfish

r7c6 is already filled with 7.

Sudoku grid illustrating step 10: Finned Swordfish — r7c6 is already filled with 7.

11

Finned Swordfish

r8c6 is already filled with 3.

Sudoku grid illustrating step 11: Finned Swordfish — r8c6 is already filled with 3.

12

Finned Swordfish

Digit 6 cannot go in r9c6 because r9c3 is in this row and r8c4 is in this box. Remove the entry or choose a different digit.

Sudoku grid illustrating step 12: Finned Swordfish — Digit 6 cannot go in r9c6 because r9c3 is in this row and r8c4 is in this box. Remove the entry or choose a different digit.

13

Finned Swordfish

r1c7 is already filled with 4.

Sudoku grid illustrating step 13: Finned Swordfish — r1c7 is already filled with 4.

14

Finned Swordfish

r3c7 is already filled with 1.

Sudoku grid illustrating step 14: Finned Swordfish — r3c7 is already filled with 1.

15

Finned Swordfish

r5c7 is already filled with 8.

Sudoku grid illustrating step 15: Finned Swordfish — r5c7 is already filled with 8.

16

Finned Swordfish

r7c7 is already filled with 2.

Sudoku grid illustrating step 16: Finned Swordfish — r7c7 is already filled with 2.

17

Finned Swordfish

r8c7 is already filled with 9.

Sudoku grid illustrating step 17: Finned Swordfish — r8c7 is already filled with 9.

18

Finned Swordfish

Digit 6 cannot go in r9c7 because r9c3 is in this row and r7c9 is in this box. Remove the entry or choose a different digit.

Sudoku grid illustrating step 18: Finned Swordfish — Digit 6 cannot go in r9c7 because r9c3 is in this row and r7c9 is in this box. Remove the entry or choose a different digit.

19

Finned Swordfish

One base column has extra candidates in box 4. That box is the fin, so this becomes a Finned Swordfish.

Sudoku grid illustrating step 19: Finned Swordfish — One base column has extra candidates in box 4. That box is the fin, so this becomes a Finned Swordfish.

20

Finned Swordfish

Because of the fin, only cells in box 4 that also lie in rows 2, 4 and 6 can lose digit 6.

Sudoku grid illustrating step 20: Finned Swordfish — Because of the fin, only cells in box 4 that also lie in rows 2, 4 and 6 can lose digit 6.

21

Finned Swordfish

These are the cells that see both the fish structure and the fin: r6c2.

Sudoku grid illustrating step 21: Finned Swordfish — These are the cells that see both the fish structure and the fin: r6c2.

22

Finned Swordfish

Remove 6 from: r6c2.

Sudoku grid illustrating step 22: Finned Swordfish — Remove 6 from: r6c2.

23

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.