Finned Swordfish

A Finned Swordfish takes the idea of the Finned X-Wing up one size, to the order-3 fish. It's a Swordfish that would be perfect but for one extra candidate — the fin — which limits the eliminations to cells near it.

Swordfish, with a fin

A clean Swordfish is a digit confined, across three rows, to the same three columns; you then clear the digit from the rest of those columns. A Finned Swordfish is the same shape with one extra candidate in one of the defining rows, breaking the tidy 3×3 pattern. That extra cell is the fin.

Which eliminations survive

Exactly as with the finned X-Wing: an elimination only holds where the target cell is attacked regardless of whether the fin turns out to be the digit. In practice that means the surviving eliminations are the cover cells (in the three columns) that also see the fin — usually the ones sharing the fin's box. So instead of sweeping three whole columns, you clear a handful of cells beside the fin.

Step by step

1. Find a near-Swordfish — a digit almost confined to three columns across three rows.
2. Identify the fin — the extra candidate spoiling the clean fish.
3. Restrict the eliminations to cover cells that also see the fin (its box).

Is it worth it?

Finned fish are genuinely useful — they occur far more often than clean ones, because a single stray candidate is common. The Finned Swordfish is the point where you're deep into expert territory, and a solver comfortable here can crack almost any logically-solvable grid. Beyond it lie Sashimi variants and the larger Jellyfish, but those are rarely needed in practice.

Common mistakes

• Over-eliminating. Keep only the cover cells that also see the fin.
• Fins in two boxes. Extra candidates spread across boxes leave no safe elimination.

Which Sudoku types is this best for?

The same logic applies across variants, but it pays off more in some than others.