Proven Sudoku
Finned Fish
Also called Fish with fins, Finned X-Wing, Finned Swordfish.
New to this shorthand? How to read a sudoku grid explains r4c7, houses, candidates and the rest.
What it means
A fish pattern with one extra square hanging off it. Without that square the pattern would be perfect. So you keep only the eliminations that work whether the extra square holds the number or not.
The exact rule, for stronger players
The rule, stated exactly: A fish where one or more base candidates lie outside the cover sets. Those are the fins. Eliminations are restricted to cover cells that also see every fin.
What it removes: Remove d from cover cells outside the bases that see every fin.
Why it works: If every fin is false the fish is exact and eliminates normally. If a fin is true, the fin cell holds the digit. A cell that both is a normal fish target and sees every fin is excluded in either case.
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
- Complex Fish (rank framework)The fish idea, but boxes are allowed to join the rows and columns.
- Basic Fish (base and cover formulation)Pick some rows where a digit has only a few spots left.
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: much more common than exact fish above size two, because the fin relaxes a strict condition.