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Jellyfish

A four-line fish: the same single-digit pattern as X-Wing and Swordfish, scaled to four rows and four columns.

Jellyfish is the four-line member of the fish family. For one digit, four rows confine its candidates to a common set of four columns; the digit is then removed from those four columns in all other rows.

Beyond Jellyfish, larger fish exist only in theory — by the 4×4 case the complement is itself a fish, so nothing new can be found above size four on a 9×9 grid.

How to spot it

Identify four rows whose candidate cells for one digit lie within four shared columns, then eliminate the digit from those columns elsewhere. Jellyfish are rare and usually only worth hunting once X-Wing and Swordfish are exhausted.

  • One digit, four rows, four shared columns.
  • Eliminate the digit from those columns in other rows.
  • Check the column-orientation mirror too.
The four-row, four-column fish: every other 7 in those columns is out.

Worked example

  1. Digit 7 across rows 1,3,6,8 is confined to columns 1,4,5,9.
  2. Together they form a jellyfish.
  3. Remove 7 from columns 1,4,5,9 in the remaining rows.
  4. The resulting eliminations usually unlock several cells.
  5. Continue with singles.

Try it yourself

3
1
7
3
9
2
5
9
4
4
5
3
9
6
7
3
9
5
8
3
5
7
8
2
9
4
5
9

Tap a cell, then a number, to practise.

Frequently asked questions

How common is a jellyfish?
Rare. Most puzzles solvable by fish are cracked by X-Wing or Swordfish first; jellyfish appears mainly in the hardest grids.
Are there fish bigger than jellyfish?
On a standard 9×9 grid, no useful ones — a size-5 fish is the complement of a size-4 fish.

Related techniques

Practice: Jellyfish

Put the Jellyfish to work on a live board — free puzzles with notes, hints and four difficulty levels.

Try it on a live board

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