Jellyfish

Overview

Tier 4

Fish, wings, chains & uniqueness

Typical difficulty

Peak Score 70–85

Also known as

4-Fish, Four-Line Fish

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.

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.

How to apply it

Spotting the pattern and making the deduction, in order.

1. 01Pick a candidate digit and find four rows where it appears in two to four cells.
2. 02Confirm that all candidate cells across all four rows share the exact same four columns.
3. 03Each of the four rows must place the digit, filling one position in each of the four columns.
4. 04The digit is fully accounted for in those four columns by the four rows.
5. 05Eliminate that candidate from all other cells in those four columns.

Common mistakes

• !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.

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