Start with the idea
Start with an almost Swordfish for one number. In this example, the number is 6 and the extra 6 is the fin. Either the Swordfish works or the fin is 6; only the red 6s that fail both possibilities 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
5
6
7
6
6
6
1
5
2
6
9
9
8
1
6
8
3
4
6
8
6
7
2
2
8
4
1
9
5
8
6
7
6
Look here firstFocus on the number 6. The three-line fish has one extra 6, and that extra note controls where removals are allowed.
This technique reviews candidates instead of solving a square right away.
Because of the fin, only the red 6s that also fit the fin-box restriction can be removed.
- Find an almost Swordfish for one number.
- Identify the fin box.
- Remove that number only where the fish line and fin box agree.
cells in the patternextra fin notenotes to remove
When to look for it
Use it when a three-line fish is almost clean but one extra note sits in a box that crosses the pattern.
How to use it
- Find a Swordfish pattern with one or more fins in a single box.
- Identify the line that passes through the fin box.
- Remove the fish digit from matching cells in that box and line.
Common mistakes
- Fins expand the pattern but narrow where removals are allowed.
- Confirm all fins fit the same fin box before removing notes.