Jellyfish
The Jellyfish is the largest of the basic fish and completes the family: the X-Wing (2×2), the Swordfish (3×3), and the Jellyfish (4×4). It's rare — most grids fall to something simpler first — but the logic is exactly the same, just one size up.
What is a Jellyfish?
Pick a digit. Find four rows in which it can go only in cells that fall within the same four columns. (As with all fish, a row may use fewer than four of those columns — it just can't stray outside them.) The four digits in those rows must occupy those four columns between them, so the digit can be removed from every other cell in the four columns.
In rows 1, 4, 6 and 9 the digit 2 is confined to columns 1, 4, 6 and 9 (amber). So 2 is eliminated everywhere else in those four columns — the red cells.
Step by step
- Choose a digit and map where it can go, row by row.
- Find four rows confined to the same four columns.
- Confirm no row breaks out of those four columns.
- Eliminate the digit from the rest of the four columns — then try it with rows and columns swapped.
Is it worth learning?
Honestly, a Jellyfish is seldom the only way through a grid — by the time a digit is this constrained, a Swordfish, wing or chain has usually already cracked it. But it rounds out the fish family, and on the occasional puzzle built to need it, nothing else will do. Note that a fish of size five or more is never necessary — a mathematical fact, so the family stops at the Jellyfish.
Which Sudoku types is this best for?
The same logic applies across variants, but it pays off more in some than others.