Finned Fish
A Fish pattern with one extra candidate — the fin — whose box makes a local elimination safe.
What it is
A Finned Fish is almost a normal X-Wing or Swordfish. Its base rows and columns still confine one digit, but one base line has an extra candidate outside the Fish. That extra candidate is the fin. When the fin and one matching Fish corner lie in the same box, the digit must be resolved inside that box in one of two ways. Candidates on the matching cross-line inside the fin box can therefore be removed.
How to spot it
Start as if you were searching for an X-Wing or Swordfish: choose one digit and list its candidate positions on the lines. Look for two or three base lines whose candidates fit in the same cross-lines, except for one extra candidate. Verify that every fin and the matching Fish corner share one box. Only then inspect that cross-line inside the fin box for eliminations.
Worked example
Step through the reasoning below. Each step adds one layer: first the position, then the pattern, then the candidates involved, and finally what can be eliminated.
Step 1 of 4: The starting position
- Pattern cells
- 5/6, 6/5, 6/6, 6/9, 7/6, 7/9
- Digits
- 5
- Eliminations
- 5/6
Press a marked square to point it out on the board above.
Why it matters
Finned Fish is useful because real expert grids rarely present a perfect rectangle. The extra candidate looks like a flaw, but the box constraint restores a precise local conclusion. It is the bridge between regular Fish and more flexible chain reasoning.
Practise now
Solve a puzzle where this technique decides the game
Frequently asked questions
How is Finned Fish different from X-Wing?
An X-Wing has no extra candidate on its two base lines. A Finned Fish has one or more fins outside the base cross-lines. The extra candidate prevents a whole-line elimination, but the shared box still permits a smaller, local elimination.
Where can I remove the candidate?
Only in the fin box, on the cross-line through the matching Fish corner, and outside the Fish base lines. Removing the digit anywhere else would overstate the proof.
Can a Finned Fish use three lines?
Yes. Sudoku Luna verifies Finned X-Wing and Finned Swordfish patterns. The larger pattern follows the same rule: all base cross-lines must fit, and the fin box must justify the exact target cells.
Put it to use
Where you will meet it
Expert grids are the only place this pattern regularly decides the solve. Bring your notes.
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