Hard Used from hard puzzles up.
Jellyfish
An X-Wing over four lines
A Jellyfish is an X-Wing with four rows and four columns: a digit whose candidates in four rows all fall within the same four columns.
The four rows need the digit four times, once in each of those columns, so the columns have it: remove it from their other cells.
A row may have the digit in only two or three of the four columns. Jellyfish are rare: most of the time, an X-Wing or a Swordfish the other way round does the same work.
What to look for
Pick a digit and look for four rows (or columns) whose candidates for it share the same four columns.
Example
- The cells the technique relies on
- Cells where it removes candidates, struck out in red
Practise it
In the app, Learn › Jellyfish sets exercises on real positions, then a full puzzle that needs this technique and nothing harder.