TECHNIQUE · EXPERT
Jellyfish — the 4×4 Fish pattern
Following X-Wing (2×2) and Swordfish (3×3), Jellyfish extends the Fish family to 4 rows and 4 columns. When a digit's candidates across four rows are confined to the same four columns, those columns can be cleared in every other row.
The logic
Digit N appears only within columns {W, X, Y, Z} across rows A, B, C, and D. Each row places N once, consuming one of the four columns. So the four columns are fully spoken for, and other rows cannot hold N in those columns.
The Fish family
An n-row pattern that confines a digit to the same n columns is called a Fish. n=2 is X-Wing, n=3 is Swordfish, n=4 is Jellyfish. Larger fish exist in theory, but Jellyfish is the deepest variant that actually appears in published puzzles.
How to find it
For one digit, list candidate columns per row, then look for any 4-row group whose column union has exactly four elements. Each row may carry 2–4 candidate columns.
- Pick an unplaced digit.
- List candidate columns per row.
- Search for a 4-row group whose column union has exactly four elements.
- Remove the digit from those four columns in all other rows.
Practice order
- Pick a digitChoose a digit with many remaining candidate cells.
- Map per rowRecord candidate columns for the digit in each row.
- Find the 4-row groupLook for four rows whose column union is exactly four.
- Eliminate columnsRemove the digit from those four columns in all other rows.
Walk through a Jellyfish
Step 1 of 4
Focus on digit 4. Across rows 1, 3, 6, and 8, the candidate cells for 4 land only inside columns 1, 4, 6, and 9.