Jellyfish Technique
advancedThe 4x4 fish pattern — the largest practical fish
Jellyfish is the 4x4 extension of the fish family. A candidate is confined to at most four columns across four rows (or vice versa), allowing eliminations in those columns.
Understanding the Concept
Select four rows where a candidate appears 2-4 times each, and all occurrences fall within the same four columns.
By the pigeonhole principle, these four rows each contribute exactly one instance of the digit to these four columns.
All four columns are therefore "claimed" by these rows. The digit can be eliminated from any other cell in those four columns.
The transposed version works too: four columns confining to four rows, eliminating from the rows.
Jellyfish is extremely rare in published puzzles. Most solvers never encounter one.
If you think you need a Jellyfish, double-check for simpler patterns first — missed X-Wings or Swordfish are far more likely.
Examples
Jellyfish on Digit 6 (Rows 0, 2, 5, 8 / Columns 1, 3, 5, 7)
Digit 6 appears in rows 0, 2, 5, and 8, confined to columns 1, 3, 5, and 7. Each pattern row has 2-3 occurrences within those columns (blue). Candidate 6 can be eliminated from all other cells in those four columns (red). Jellyfish are extraordinarily rare in practice.
Pro Tips
- •Jellyfish is extraordinarily rare — most published puzzles never require it
- •If you suspect you need one, first re-check for missed X-Wings and Swordfish
- •The logic is identical to X-Wing and Swordfish, just one dimension larger
- •Larger fish (5x5 and beyond) exist in theory but are never needed in practice
- •Understanding the fish family conceptually matters more than memorizing Jellyfish specifically
