Jellyfish
The four-line fish. A candidate confined to four columns across four rows is eliminated from the rest of those columns.
Candidate 8 in four rows is confined to just four shared columns.
How to spot it
- 1Find four rows where a candidate appears in two to four cells.
- 2Confirm all sit within the same four columns.
- 3Remove the candidate from the columns' other cells.
When to use
Almost never by hand — it appears only in extreme puzzles.
When not to
If you're reaching for a jellyfish, a forcing chain is usually faster.
Common mistake
Mis-tracking one of the sixteen possible positions and drawing a false pattern.
Worked Example
A jellyfish is the four-line version of the same fish family: X-Wing uses two rows and two columns, swordfish three, jellyfish four. If candidate 8 across four rows is confined to cells that only ever fall in four shared columns, those columns must supply their 8 from within the four rows.
Every other 8 candidate in those columns, outside the four rows, can then be eliminated. In practice this pattern is rare and easy to misread — with sixteen possible cell positions to track, it's worth double-checking the columns really do align before committing to the elimination.