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Finned & Sashimi Fish
Learn finned and Sashimi fish: an X-Wing or Swordfish that almost works but for a few extra candidates in one box, still forcing eliminations that see every fin.
A finned fish is a fish — an X-Wing, Swordfish or larger — that would be perfect except for one or a few extra candidates, called the "fins", sitting in a base line outside the cover lines. A pure fish needs the digit confined to exactly the cover lines; the fins are what stops that.
The trick is that the fins are all trapped inside a single box. So reason in two cases: if the fish is real (the fins turn out empty of the digit), the normal fish eliminations apply; if instead the digit lands on a fin, it sits in that box. Any cell that is both a normal fish victim and can see every fin is therefore impossible either way — and only those cells are eliminated.
A Sashimi fish is the same idea where one cover line is so depleted it relies entirely on the fin; the logic is identical. Finned fish are common on expert boards where a clean X-Wing is "almost" there, and spotting the fin turns a near-miss into a real elimination. The example highlights the fish lines and fin, and the candidate they remove.
Practise the Finned X-Wing
The best way to learn a technique is to use it. Play a puzzle at the level where it first appears, or drop a tricky board into the solver to watch it in action.
Frequently asked questions
What is a finned fish in Sudoku?
A fish pattern (X-Wing, Swordfish…) with extra "fin" candidates in a base line, all confined to one box. Cells that are both ordinary fish victims and see every fin can have the digit removed.
What is the difference between finned and Sashimi?
They follow the same logic. "Sashimi" is the case where removing the fin would leave a cover line with too few candidates to be a real fish on its own — the fin is doing the work. Most solvers treat them together.
Why do the fins have to share a box?
The elimination only holds for cells that see the digit no matter where it goes. If every fin sits in one box, a single cell can see them all at once, which is what makes the deduction valid.
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