Start with the idea
Start with an almost Jellyfish for one number. In this example, the number is 2 and the extra 2 is the fin. Either the Jellyfish works or the fin is 2; the red 2 fails both possibilities, so it can be removed.
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
2
3
2
7
2
6
1
9
5
2
4
7
2
9
8
2
2
2
8
6
3
4
8
3
1
7
2
2
2
6
8
2
4
1
9
2
2
8
7
Look here firstFocus on the number 2. The four-line fish has an extra fin, so the removal must stay inside the fin's restricted area.
This technique reviews candidates instead of solving a square right away.
The red 2 fits the fin-box restriction, so it cannot be correct.
- Find an almost Jellyfish for one number.
- Identify the fin or fins.
- Remove that number only from cells allowed by the fin restriction.
cells in the patternextra fin notenotes to remove
When to look for it
Use it only on very advanced boards after simpler fish and wing checks do not help.
How to use it
- Find a four-line fish pattern for one digit.
- Identify the extra fin note or notes.
- Remove the digit only from cells that fit the fin-box restriction.
Common mistakes
- This is one of the easiest patterns to misread visually.
- If the removal is not in the fin box, re-check the pattern.