A digit confined to 2-4 cells in each of four rows, all within four columns. The largest standard fish pattern.
A Jellyfish is the 4×4 extension of the fish pattern family: X-Wing (2×2), Swordfish (3×3), Jellyfish (4×4). A single digit appears in 2-4 cells in each of four rows, and all those cells fall within the same four columns. Since each of the four rows must place the digit in one of those four columns, the digit is "claimed" by those rows and can be eliminated from any other cell in those four columns.
In practice, Jellyfish is vanishingly rare. Most Extreme puzzles that seem to need Jellyfish can be solved with combinations of simpler techniques applied first. When a true Jellyfish does appear, it can eliminate several candidates at once.
If you understand X-Wing and Swordfish, the Jellyfish logic is identical — just extended to four base sets. The example below walks through a complete Jellyfish pattern step by step.
Pick a digit. Find four rows where that digit appears as a candidate in 2-4 cells each, and all those cells fall within exactly four columns. (Or equivalently, four columns whose candidates all fall within four rows.)
The four rows must each place the digit somewhere within those four columns. That accounts for all four column slots, so no other row can use any of those columns for the digit.
Eliminate the digit from every other cell in those four columns that isn't in one of the four defining rows. The same logic works transposed: if you find the pattern in columns, eliminate from the corresponding rows.
Step 1: Scanning digit 4, we find four rows with limited positions. In row 1, digit 4 appears in R1C4 and R1C6. In row 4, it appears in R4C4, R4C5, and R4C6. In row 7, it appears in R7C5 and R7C6. In row 8, it appears in R8C2, R8C4, and R8C6. All positions fall within columns 2, 4, 5, and 6.
Step 2: This is a Jellyfish. Digit 4 must occupy exactly one cell from each defining row within columns 2, 4, 5, and 6. Therefore, 4 can be eliminated from all other cells in those four columns that are not in the defining rows.
Step 3: Eliminate 4 from R2C2, R2C4, R2C6, and R3C2. The Jellyfish locks digit 4 into the four defining rows across four columns.
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