Sudoku
Jellyfish practice
A Jellyfish is the four-line fish: one number, four rows, and only four columns between them. Each row can use two, three or four of those columns.
The four rows account for the number in all four columns, so it can be crossed off the rest of each column. The same holds with columns and rows swapped.
How to spot a Jellyfish
A Jellyfish is rare, and a simpler step is usually available first. Look for it when one number still has many candidates and nothing else moves: count the rows where it has four or fewer places and see whether four of them share four columns.
Practise it on this page
The board above deals positions where the Jellyfish is the only way forward, with every pencil note filled in. Cross off the candidates it rules out and press Next. Press Hint to see the pattern marked on the board.
In a full game, Extreme is the first level that can ask for it.