Jellyfish
Four rows where a digit can only fit within the same four columns. Eliminate that digit from the rest of those columns. The pattern is symmetric — four columns × four rows works the same way.
Advanced sudoku technique
What it is
Jellyfish extends Swordfish by one more row and column. Pick a digit. Find four rows in which that digit's only candidate cells lie within the SAME four columns (a row can have the digit in only two or three of those four columns — that's still in-pattern, as long as it stays inside the four-column set). Each of those rows must place the digit somewhere in those four columns. The four placements distribute one per row × one per column across the 4×4 grid of corners. Therefore the digit cannot appear in those four columns in any OTHER row. The column-rooted variant is symmetric. Jellyfish is the largest fish that appears in standard sudoku puzzles — a fifth-order fish (Squirmbag) would by mathematical necessity reduce to a smaller fish on the OTHER four-row group, so it's redundant.
When to use it
On master and evil tier puzzles when X-Wing and Swordfish are exhausted. Very rare in practice — most expert-tier puzzles never need it.
Worked example
Digit 3's candidate cells in rows 1, 4, 7, 9 all lie within columns 2, 5, 6, 8. The four placements must distribute one per row × one per column across that 4×4 grid. Therefore 3 can be eliminated from columns 2, 5, 6, 8 in rows 2, 3, 5, 6, 8.
Try it
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