Jellyfish
The 4-row / 4-column version of Swordfish.
Also known as: 4-Fish, Four-Line Fish
Quick Summary
When a candidate in four rows is confined to a union of four columns, that candidate can be eliminated from all other cells in those four columns.
Understanding the Jellyfish Technique
Jellyfish is a 4-fish—the largest basic fish pattern relevant to standard 9×9 Sudoku. If a digit in four rows is confined to four columns, those four columns are saturated with that digit, eliminating it from all other cells in those columns. A 5-fish is mathematically equivalent to a 4-fish on the remaining 4 lines, making Jellyfish the apex of standard fish logic.
Visual Examples
Green cells highlight the Jellyfish pattern, amber cells denote where candidates are affected, and struck-through pencil marks show the eliminations. Use the buttons or arrow keys to browse examples.
How to Spot & Solve a Jellyfish (Step-by-Step Guide)
The complete visual scanning and logical deduction process to execute this technique:
- 1Pick a candidate digit and find four rows where it appears in two to four cells.
- 2Confirm that all candidate cells across all four rows share the exact same four columns.
- 3Each of the four rows must place the digit, filling one position in each of the four columns.
- 4The digit is fully accounted for in those four columns by the four rows.
- 5Eliminate that candidate from all other cells in those four columns.
Common Traps & Mistakes to Avoid
- Missing an extra candidate in one row that breaks the 4-column boundary.
- Confusing a Jellyfish with smaller fish patterns that could solve the board faster.
Frequently Asked Questions
Why isn't there a 5-fish (Starfish)?
Because in a 9x9 grid, any 5-line fish is mathematically the inverted complement of a 4-line Jellyfish on the other 4 lines.
Related Solving Techniques
Expand your repertoire by learning prerequisites and advanced counterparts:
Ready to Practice Jellyfish?
Try a puzzle in the Expert tier (Peak Score 71–80), or browse all 25 techniques.