Start with the idea
Start with an almost X-Wing for one number. In this example, the number is 5 and the extra 5 is the fin. Either the X-Wing works or the fin is 5; in both cases, the red 5s in the fin's box cannot be correct.
Look for this pattern
Look for a fish pattern with one extra candidate near the removals.
This empty board keeps the puzzle numbers out of the way so the pattern is easier to see.
Example
3
7
2
5
6
5
1
9
5
5
8
6
1
5
9
8
6
3
4
8
3
7
2
6
6
5
2
5
4
1
9
8
7
Look here firstFocus on the number 5. The pattern is almost an X-Wing, but one extra 5 sits in the same box as the removals.
This technique reviews candidates instead of solving a square right away.
Either the X-Wing works or the fin is 5. In both cases, the red 5s in the fin's box cannot be correct.
- Find an almost X-Wing for one number.
- Identify the extra fin in one box.
- Remove that number only from matching cells inside the fin's box.
cells in the patternextra fin notenotes to remove
When to look for it
Use it when an X-Wing has one extra note in one pattern row or column, and that extra sits in a useful box.
How to use it
- Find an X-Wing shape with one extra note in a pattern row or column.
- Identify the fin's box.
- Remove the fish digit from matching cells inside that fin box.
Common mistakes
- The fin must belong to the same row or column as the fish pattern.
- Removals are limited to cells that line up with the fish and sit in the fin's box.