Finned X-Wing

Level 8 · Blue Belt

Strategy:

Finned & Sashimi Fish

Requires:

Learn after:

Finned Swordfish

Overview

A Finned X-Wing is an X-Wing with extra candidates (the "fin") in one of the base rows/columns. The fin restricts eliminations to cells that see both the fin and the normal elimination zone.

How It Works

Find an X-Wing pattern where one row has extra occurrences of the candidate.
These extra cells are the "fin".
Eliminations occur only in cells that see both the X-Wing column and the fin.

Example

Starting position

Look for Finned X-Wing.

Sudoku grid showing the initial puzzle state for Look for Finned X-Wing.

1

Columns 1 and 6 form a X-Wing on 4

Base columns 1 and 6 form a X-Wing for digit 4 in cover rows 1 and 6.

Sudoku grid illustrating step 1: Columns 1 and 6 form a X-Wing on 4 — Base columns 1 and 6 form a X-Wing for digit 4 in cover rows 1 and 6.

2

4 is blocked from r2c1 by the row

Digit 4 cannot go in r2c1 because r2c9 is already in this row. Remove the entry or choose a different digit.

Sudoku grid illustrating step 2: 4 is blocked from r2c1 by the row — Digit 4 cannot go in r2c1 because r2c9 is already in this row. Remove the entry or choose a different digit.

3

r3c1 is already filled

r3c1 is already filled with 7.

Sudoku grid illustrating step 3: r3c1 is already filled — r3c1 is already filled with 7.

4

r4c1 is already filled

r4c1 is already filled with 3.

Sudoku grid illustrating step 4: r4c1 is already filled — r4c1 is already filled with 3.

5

r5c1 is already filled

r5c1 is already filled with 6.

Sudoku grid illustrating step 5: r5c1 is already filled — r5c1 is already filled with 6.

6

4 is blocked from r7c1 by the row

Digit 4 cannot go in r7c1 because r7c4 is already in this row. Remove the entry or choose a different digit.

Sudoku grid illustrating step 6: 4 is blocked from r7c1 by the row — Digit 4 cannot go in r7c1 because r7c4 is already in this row. Remove the entry or choose a different digit.

7

r8c1 is already filled

r8c1 is already filled with 5.

Sudoku grid illustrating step 7: r8c1 is already filled — r8c1 is already filled with 5.

8

r9c1 is already filled

r9c1 is already filled with 9.

Sudoku grid illustrating step 8: r9c1 is already filled — r9c1 is already filled with 9.

9

r2c6 is already filled

r2c6 is already filled with 1.

Sudoku grid illustrating step 9: r2c6 is already filled — r2c6 is already filled with 1.

10

r3c6 is already filled

r3c6 is already filled with 2.

Sudoku grid illustrating step 10: r3c6 is already filled — r3c6 is already filled with 2.

11

r4c6 is already filled

r4c6 is already filled with 7.

Sudoku grid illustrating step 11: r4c6 is already filled — r4c6 is already filled with 7.

12

4 is blocked from r7c6 by the row

Digit 4 cannot go in r7c6 because r7c4 is already in this row. Remove the entry or choose a different digit.

Sudoku grid illustrating step 12: 4 is blocked from r7c6 by the row — Digit 4 cannot go in r7c6 because r7c4 is already in this row. Remove the entry or choose a different digit.

13

r8c6 is already filled

r8c6 is already filled with 3.

Sudoku grid illustrating step 13: r8c6 is already filled — r8c6 is already filled with 3.

14

r9c6 is already filled

r9c6 is already filled with 5.

Sudoku grid illustrating step 14: r9c6 is already filled — r9c6 is already filled with 5.

15

The fin sits in box 5

One base column has extra candidates in box 5. That box is the fin, making this a Finned X-Wing.

Sudoku grid illustrating step 15: The fin sits in box 5 — One base column has extra candidates in box 5. That box is the fin, making this a Finned X-Wing.

16

Only box 5 can lose 4

Because of the fin, only cells in box 5 that also lie in rows 1 and 6 can lose digit 4.

Sudoku grid illustrating step 16: Only box 5 can lose 4 — Because of the fin, only cells in box 5 that also lie in rows 1 and 6 can lose digit 4.

17

Cells that see both fish and fin

These cells see both the fish structure and the fin, so they are removal targets: r6c5.

Sudoku grid illustrating step 17: Cells that see both fish and fin — These cells see both the fish structure and the fin, so they are removal targets: r6c5.

18

Remove 4 from the fin-affected cells

Remove 4 from these fin-affected cells: r6c5.

Sudoku grid illustrating step 18: Remove 4 from the fin-affected cells — Remove 4 from these fin-affected cells: r6c5.

19

Result

Result after applying the technique.

Sudoku grid showing the result after applying the technique

Tips & Patterns

The fin must be in the same block as one of the proper X-Wing cells.
Finned fish reduce the elimination zone but still allow some progress.
These appear when you almost have a perfect fish pattern.