Proven Sudoku
Endo-Fin
Also called Endofin, Internal fin.
New to this shorthand? How to read a sudoku grid explains r4c7, houses, candidates and the rest.
What it means
A spare square that sits inside two parts of the pattern at the same time. It breaks the count in its own way. So it gets its own mark on the picture.
The exact rule, for stronger players
The rule, stated exactly: A candidate lying in the intersection of two base sets. Because it would be counted twice, it behaves as a fin and restricts eliminations the same way.
What it removes: Restrict eliminations to cells seeing every endo-fin as well as every ordinary fin.
Why it works: The counting argument assumes base sets are disjoint in their candidates. An overlap breaks the count by one, exactly as an escaping candidate does, so the same conditional applies.
Worked examples
None yet. This pattern does not turn up in our daily boards, so there is no real example to show. We would rather wait than draw a fake one.
Learn these first
- Finned FishA fish pattern with one extra square hanging off it.
Related complex fish
- Franken FishA fish where a box is allowed to join in with the rows and columns.
- Kraken FishA fish that does not quite work.
- Mutant FishA fish where rows, columns and boxes can be mixed freely on both sides.
- Sashimi FishA finned fish that is missing a corner.
- CannibalismWhen a square belongs to both halves of the fish at once.
How often does it come up?
Our rough estimate: appears whenever base sets overlap, which is common in Franken and Mutant fish.