Jellyfish is the four-row sibling of X-wing and swordfish. It is the largest fish a human solver ever needs. Anything bigger collapses into a smaller fish you already had.

How it works

Pick a digit. Find four rows where the digit has two, three or four candidate cells. If every candidate cell across those four rows sits inside the same set of four columns, you have a jellyfish.

Each of the four rows must place the digit somewhere in the jellyfish. The four placements distribute across the four columns, one per row and one per column. So the digit cannot appear anywhere else in those four columns. Erase it from every other cell in them.

Four rows with candidates for 3 confined to four columns. The rest of those columns can be cleared.

When to look for it

After X-wing and swordfish searches come up empty. In practice, jellyfish is extremely rare in puzzles solvable without chains. If you think you see one, check whether two overlapping X-wings or a single swordfish would have produced the same eliminations.

Some "jellyfish" puzzles are actually solvable with simpler techniques. Sweep the basics one more time before committing to a jellyfish search.

Step-by-step example

Pick a digit. List the rows where the digit has two, three or four candidate cells.
Pick four such rows. Write down the union of columns carrying candidates across those rows.
If the union is exactly four columns, you have a jellyfish.
Erase the digit from every cell in those four columns outside the four rows.
Re-scan. The eliminations are usually dramatic when a real jellyfish fires.

Tips for spotting it

Track digits with three or four candidate cells per row. Those are the only fish candidates.
Run the search in columns too. Four columns sharing four rows works the same way.
If you see two overlapping X-wings on the same digit, you already have the eliminations a jellyfish would give you.
Pencil marks must be flawless. A stray candidate breaks the pattern silently.

Common mistakes

Adding a row with five candidate cells. Jellyfish caps at four per row.
Mismatched column sets. All four rows must share the same four columns exactly.
Reaching for jellyfish before exhausting XY-wing and coloring. Those usually do the job.
Spending forever on a search that an overlapping pair of X-wings would have solved.

Practise it

Jellyfish is so rare that the best practice is studying example puzzles known to require it, then transferring the search habit to evil sudoku. Most expert players find a jellyfish once a year. Treat it as a curiosity, not a daily tool.