Jellyfish
The four-line fish. A digit restricted to four rows and four columns clears those columns everywhere else.
What it is
A Jellyfish extends the fish family to four lines: four rows in which a digit's candidates all fall within the same four columns. The four columns are then claimed by the four rows, and the digit can be removed from all other cells in them. Each row needs between two and four candidate cells.
How to spot it
Jellyfish are hard to find by eye. The practical approach is to work one digit at a time, list the candidate columns for every row, and look for four rows whose combined columns number exactly four. Most solvers only attempt this when a puzzle has resisted everything else.
Worked example
Step through the reasoning below. Each step adds one layer: first the position, then the pattern, then the candidates involved, and finally what can be eliminated.
Step 1 of 4: The starting position
- Pattern cells
- 1/2, 1/5, 1/6, 3/2, 3/4, 3/5, 3/6, 5/4, 5/5, 5/6, 9/4, 9/6
- Digits
- 5
- Eliminations
- 4/5, 4/4
Press a marked square to point it out on the board above.
Why it matters
A Jellyfish is the largest fish worth learning. It appears rarely, but on extreme puzzles it can be the only non-chaining move available. Knowing it also completes your understanding of why the fish family stops here.
Frequently asked questions
Why is there no five-line fish?
There is in theory, but it is never needed. In a 9×9 grid a five-line fish always has a complementary smaller fish among the remaining four lines, so you would have found that first.
Should beginners learn Jellyfish?
No. Master X-Wing and Swordfish first, and learn the wings and colouring techniques — they are far more useful in practice and appear much more often.
Put it to use
Where you will meet it
Expert grids are the only place this pattern regularly decides the solve. Bring your notes.
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