The Jellyfish Technique in Sudoku
The Jellyfish is the largest of the everyday “fish” techniques. It is the same single-digit logic as the X-Wing and Swordfish, stretched to four rows and four columns. It is rare and only matters on the very hardest grids, but it needs no new idea once you can read its smaller cousins.
How to use a Jellyfish, step by step
- Work from full pencil marks and pick one digit.
- Find four rows where the digit appears in only two, three or four cells each.
- Check that all those candidates fall within the same four columns.
- If they do, the digit is locked to that 4×4 frame.
- Eliminate the digit from those four columns everywhere outside the frame.
What is a Jellyfish?
A Jellyfish confines one digit to four columns across four rows (or vice versa), so it can be removed from the rest of those columns. It completes the fish ladder: the X-Wing uses two lines, the Swordfish three, the Jellyfish four. Beyond four, larger fish are always covered by simpler moves, so the Jellyfish is the practical end of the family.
How to spot a Jellyfish
It is hard to find by eye — most solvers meet the Jellyfish through a step-by-step solver rather than spotting it raw. If you do hunt one, track a single digit and look for four rows whose candidates all live inside four columns. It is genuinely rare; reach for it only when everything simpler has failed. See the full toolkit in tips & strategy.
Jellyfish in Sudoku: FAQ
What is a Jellyfish in Sudoku?
A Jellyfish is a single-digit fish technique where a digit is confined to four columns across four rows. Those intersections use up the digit in each column, so it can be eliminated from the rest of those columns.
How is a Jellyfish related to the X-Wing and Swordfish?
They are the same idea at different sizes: the X-Wing spans two lines, the Swordfish three and the Jellyfish four. Each locks a digit to a square frame of intersections and clears it from the rest of those lines.
How often do you need a Jellyfish?
Very rarely — only on the hardest grids, and even then simpler moves usually appear first. Many solvers only ever see one via a step-by-step solver.