How it works
Jellyfish is the last of the fish family solvable by humans. Five-fish patterns exist mathematically but do not produce eliminations a smaller fish has not already produced.
The conditions: four rows, each with two, three or four candidate cells for the chosen digit, all confined to the same four columns. The digit then commits to four of the sixteen possible cells, one per row and one per column.
When a jellyfish applies, the eliminations are usually dramatic. The catch is that jellyfish patterns are extremely rare in puzzles solvable without forcing chains.
When to look for it
After X-wing and swordfish. Jellyfish is rare in human-rated puzzles and often signals you have missed an easier technique elsewhere.
Tips for spotting the pattern
- If you think you see a jellyfish, sanity check with X-wing and swordfish first. They usually cover the same eliminations.
- Track digits with three or four candidate cells per row. Those are the only fish candidates.
- Some "jellyfish" are actually two overlapping X-wings. Either way, the eliminations work.
Common mistakes
- Adding rows with five or more candidates. Jellyfish breaks above four.
- Mismatched column sets. All four rows must constrain to the same four columns.
- Skipping it for chain techniques. Jellyfish is cheaper than coloring or forcing chains when it applies.