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Compare advanced Sudoku fish techniques
Use this one-page guide to decide whether a candidate pattern is an X-Wing, Swordfish, finned fish or Jellyfish before making an elimination.
The proof changes with the size and the fin
| Pattern | Base lines | Cover lines | Extra candidate | What must be proved |
|---|---|---|---|---|
| X-Wing | 2 | 2 | None | Every base-line candidate lies on the same two cover lines. |
| Swordfish | 3 | 3 | None | Every base-line candidate lies on the same three cover lines. |
| Finned X-Wing | 2 | 2 | Fin in one box | The target sees the confined fin and the affected fish corner. |
| Finned Swordfish | 3 | 3 | Fin or fins in one box | The target lies on the affected cover line and sees every possible fin. |
| Jellyfish | 4 | 4 | None | Every base-line candidate lies on the same four cover lines. |
Safe solving checklist
- Choose one candidate digit and map every occurrence in the proposed base lines.
- Confirm the number of cover lines matches the pattern size.
- For a finned pattern, confirm every fin is confined to the same 3×3 box.
- Remove a candidate only after proving the target sees every required endpoint or fin.
- If a simpler single, pair or regular fish is available, use that proof first.
Continue beyond fish patterns
When a fish does not produce a valid elimination, move to the next technique family instead of forcing the pattern.