Finned Swordfish Sudoku Technique

Updated 11 August 2026

A Finned Swordfish is a Swordfish pattern with one extra candidate cell, called a fin, sitting outside the three cover columns, which narrows the elimination down to whatever the fin itself can still see.

When a Finned Swordfish applies

A plain Swordfish needs three rows whose candidate cells for a digit span exactly three columns between them. A Finned Swordfish starts from the same three rows, but one of them carries a fourth, stray candidate cell outside the shared three columns. That stray cell is the fin, and just as with the Finned X-Wing, it does not automatically break the pattern — it narrows what the pattern can safely prove.

The reasoning is the same two-branch argument as the smaller finned fish. If the fin holds the digit, any cell that shares a unit with the fin cannot. If the fin does not hold the digit, the three rows collapse back into a clean Swordfish, and any cell inside the three cover columns cannot hold the digit either. A cell that satisfies both branches — inside a cover column and visible to the fin, most often because it shares the fin’s box — is safe to clear regardless of which branch turns out to be true.

Because the fin restricts the safe cells to only those that also see it, a Finned Swordfish usually proves far less than a clean one would. It is still worth checking, because it can surface eliminations that a strict three-by-three frame never would on a real board.

Worked example

Across rows 6, 7 and 8, the digit 1 is a candidate at r6c6, r6c7, r7c1, r7c6, r8c1 and r8c6 — six cells spanning columns 1, 6 and 7, which is a clean Swordfish shape. Row 8 also carries 1 at r8c8, a seventh cell in a fourth column. That extra candidate is the fin: it stops the pattern from clearing 1 across the whole of columns 1, 6 and 7, because row 8 might place its 1 at the fin instead of at r8c1 or r8c6.

r8c8 sits in the bottom-right box together with r9c7. r9c7 was carrying 1 and 5 before the deduction. If the fin at r8c8 holds 1, r9c7 cannot, because they share a box. If the fin does not hold 1, rows 6, 7 and 8 fall back to the clean Swordfish on columns 1, 6 and 7, and r9c7 cannot hold 1 for that reason instead, since column 7 is one of the cover columns. Either way 1 is impossible at r9c7, so the solver removes it and leaves 5 behind — a naked single for the next pass.

How to spot a Finned Swordfish

1. Look for an almost-SwordfishFind three rows where a digit’s candidates mostly line up in the same three columns, but the combined span reaches a fourth column through exactly one extra cell.
2. Identify the finThe extra candidate cell is the fin. If more than one cell sits outside the three shared columns, the pattern does not qualify as a Finned Swordfish.
3. Find cells that see both a cover column and the finLook at the cells the ordinary Swordfish would have cleared, and keep only the ones that also share a row, column or box with the fin.
4. Eliminate the digit from those cells onlyRemove the digit from that narrower set of cells. Cells that see a cover column but not the fin are not provably clear.

Common mistakes

• Eliminating the digit across the whole cover columns, as a clean Swordfish would. A finned pattern only clears cells that also see the fin.
• Missing the fin entirely and dismissing the near-Swordfish as invalid. One extra candidate cell does not rule the pattern out, it just changes what it can prove.
• Allowing two or more stray cells and still calling it a Finned Swordfish. Beyond one fin, the argument needs every fin cell to share a box for the elimination to hold.
• Forgetting box visibility. Fin-based eliminations most often depend on a shared box, not just a shared row or column.