Jellyfish
Prerequisites: Swordfish
Overview
A Jellyfish extends the fish pattern family to four rows and four columns. When a candidate digit is restricted to at most four columns across four rows, it can be eliminated from those columns in all other rows.
How It Works
Find four rows where a candidate digit appears in 2-4 positions each, and all positions fit within exactly four columns. Each row must contain the digit once, so the four rows place four of them, no two in the same column: one in each of those four columns. Therefore, the digit cannot appear in those four columns in any other row.
When To Use It
Look for Jellyfish when X-Wing and Swordfish don't apply. Scan for a digit that has limited positions (2-4 per row) across four rows sharing four columns.
How To Detect It
For each digit, find rows with 2-4 candidate positions. Take combinations of four rows and check if all column positions fit in exactly four columns. If so, eliminate the digit from those columns outside the four rows.
Visual Example
Step 1 of 10
The Fish Family
Jellyfish is the largest fish you'll need: X-Wing (2×2) → Swordfish (3×3) → Jellyfish (4×4). Each extends the same logic to one more row and column. A Jellyfish uses four rows and four columns.
Candidates already ruled out by earlier techniques aren't shown.
Use it in a real game
You'll rarely need this one, but Hard puzzles are where it can turn up.
Play a Hard puzzle