Finned Swordfish

Prerequisite: Swordfish · Finned X-Wing

Description

Finned Swordfish is an “almost Swordfish”: the green body confines digit d to a 3×3 net, but on one base line (a row/column) there are extra candidates of the same digit — these extras are the fins (yellow).

The logic is the same idea as Finned X-Wing: consider two cases — a fin is true or no fin is used.
In practice, the safe eliminations are usually:

the overlap between “eliminations from the fin’s box” and “eliminations from the standard Swordfish”.

Finned Jellyfish (a finned 4th-order fish) follows the same reasoning — it’s just a larger net.

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Explanation

In the image above, the target digit is 1.

• Green cells: rows 3, 6, 7 form the Swordfish body (the net)
• Yellow cells: r3c5 and r3c6 in box 2 are the fins
• Red cell: we want to eliminate candidate 1 in r2c4

Why can we remove candidate 1 in r2c4? Two cases are enough:

1. If any fin is true (r3c5=1 or r3c6=1)
   Then box 2 already contains a 1, so other 1 candidates in that box are impossible — r2c4’s 1 is eliminated.

2. If all fins are false (r3c5≠1 and r3c6≠1)
   Then in rows 3, 6, and 7, digit 1 is confined to the green net again, so the pattern reduces to a standard Swordfish.
   Standard Swordfish eliminates digit 1 candidates on the cover lines outside the net — which includes r2c4’s 1.

Either way, candidate 1 in r2c4 is eliminated, so it is a safe elimination.

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Examples

This image is another finned Swordfish example. Use it as a reference for body (green) / fins (yellow) / elimination (red):

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How to Find Finned Swordfish

One-line checklist: find a Swordfish net, spot extra fin candidates on one base line, then eliminate only where the two conclusions overlap.

In a real puzzle:

1. Pick a digit d
2. Find the Swordfish body: 3 base lines where all d candidates stay within the same 3 cover lines
3. Check for extra d candidates on one base line (fins), and confirm all fins are in the same box
4. Prefer eliminations in the overlap: cells inside the fin’s box that are also eliminated by the standard Swordfish (cover-line outside the net)