Jellyfish is a size‑4 fish pattern, generalizing X‑Wing (size 2) and Swordfish (size 3). For a digit d, if four rows restrict d to the same four columns (or vice versa), eliminate d from all other cells in those columns (or rows).
When to look for it
- Hard/Expert-level puzzles where simpler techniques stall.
- Candidate notes are required—enable notes first.