A Swordfish with an extra candidate cell, the fin. Eliminations are restricted to cells seeing both the Swordfish columns and the fin's box.
Look for an almost-Swordfish: a digit confined to two or three cells in each of three rows, all within three columns, but with one extra candidate cell — the fin — sitting outside those columns. You can still eliminate the digit, but only from cells in the three columns that also share a box with the fin.
A Finned Swordfish is to a regular Swordfish what a Finned X-Wing is to a regular X-Wing. You have an almost-perfect Swordfish pattern (a digit confined to 2-3 cells in each of three rows, all within three columns) but one row has an extra candidate position (the fin) that breaks the clean pattern.
Despite the fin, useful eliminations remain. The logic considers two cases: either the fin cell doesn't contain the digit (and the Swordfish works normally), or it does (eliminating from the fin's peers). Cells eliminated in both cases are safe targets, specifically, cells in the Swordfish's columns that also see the fin cell.
Finned Swordfish is more common than regular Swordfish because the pattern requirements are looser. It appears in hard Extreme puzzles and is worth recognizing even if you don't actively scan for it.
Find three rows where a digit appears in 2-3 cells each, and those cells fall within three columns, except one row has an extra cell outside those columns. That extra cell is the fin.
If the fin is removed, you would have a standard Swordfish and could eliminate the digit from all other cells in those three columns. The fin restricts this: only cells in those columns that also share a box with the fin can be eliminated.
Apply the same two-case logic as Finned X-Wing: Case 1 (fin is off) = normal Swordfish eliminates from the columns. Case 2 (fin is on) = fin eliminates from its peers. The intersection of both cases gives you safe eliminations.
Example 1: Finned Swordfish
Scanning digit 1, columns 1, 5, and 7 almost form a Swordfish: their 1-candidates fall in rows 1, 7, and 8 — except column 7 carries one extra candidate at R3C7, the fin. The base cells are R7C1, R8C1 (col 1), R1C5, R7C5 (col 5), and R1C7, R8C7 (col 7).
Without the fin, this Swordfish would remove 1 from cover rows 1, 7 and 8 in every non-base column. The fin restricts the eliminations to cells that also see R3C7 — the only one is R1C9, which sits in cover row 1 and shares the fin's box.
Eliminate digit 1 from R1C9. Either the fin R3C7 is 1 (removing 1 from R1C9 in the same box) or it is not (and the clean Swordfish removes it) — either way, R1C9 cannot be 1.
Because the fin might hold the digit, which breaks the clean Swordfish. The logic splits in two: if the fin is empty of the digit, the Swordfish eliminates across all three columns; if the fin holds it, only the fin’s peers lose it. Only cells that lose the digit in both cases are safe to eliminate.
The fin is the extra candidate cell for the digit that sits in one of the three defining rows but outside the three columns. Everything else matches a standard Swordfish. Remove the fin mentally and you would have a clean 3×3 fish; its presence is what limits where you can eliminate.
No, it is more common, because the requirements are looser. A plain Swordfish demands every candidate fall inside the three columns, while a Finned Swordfish tolerates one stray cell. The trade-off is a much smaller elimination zone — in the worked example, a single cell.
A Finned Swordfish is the same idea one size larger. A Finned X-Wing is an X-Wing (two rows, two columns) with a stray candidate; a Finned Swordfish is a Swordfish (three rows, three columns) with a stray candidate. Both use the identical two-case argument, and both restrict eliminations to cells sharing the fin’s box.
Yes, a Finned Swordfish works with columns as the base sets and rows as the covers. The worked example is column-based: digit 1 is confined to rows 1, 7 and 8 within base columns 1, 5 and 7, with the fin at R3C7 outside those cover rows. Eliminations land in a cover row sharing the fin’s box — here R1C9.
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