sudokly

Jellyfish

A jellyfish is the four-row generalisation of X-wing (and swordfish). It applies when a digit's candidates in four rows share exactly four columns.

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.

JELLYFISH ON 6 (ROW BASE)382456745679116496734683615615949351467867678234672845699612724678914564563
Digit 6 is locked to 4 rows → erase 6 from the crossing columns.

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

  1. Pick a digit. List the rows where the digit has two, three or four candidate cells.
  2. Pick four such rows. Write down the union of columns carrying candidates across those rows.
  3. If the union is exactly four columns, you have a jellyfish.
  4. Erase the digit from every cell in those four columns outside the four rows.
  5. 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.

Put Jellyfish into practice

Play expert sudoku →Rate a puzzle's difficultyCheck your solution

Jellyfish FAQ

What is a Jellyfish in sudoku?
A Jellyfish is a single-digit fish pattern that uses four base sets and four cover sets, making it one size larger than a Swordfish. You find four rows in which a candidate is restricted to cells within the same four columns; since the digit must appear in each of those rows and only within those columns, it can be eliminated from the rest of those four columns. It applies with rows and columns swapped as well.
What is the difference between a Swordfish and a Jellyfish?
The difference is size: a Swordfish is a 3x3 fish using three base lines and three cover lines, while a Jellyfish is a 4x4 fish using four of each. A Jellyfish allows each base line to hold two, three, or four occurrences of the candidate, as long as everything stays inside the four cover lines. Jellyfish are rarer and harder to spot because of the extra line involved.
Are fish larger than a Jellyfish worth learning?
Generally no. In a standard 9x9 sudoku, any fish of size five or larger for a digit has a complementary fish of size four or smaller (its cover sets viewed as base sets), so it can always be found as a Jellyfish or smaller instead. Because of this complement rule, X-Wing, Swordfish, and Jellyfish cover every useful basic fish, and bigger fish add no new eliminations.
Why are Jellyfish so rare in sudoku?
A Jellyfish requires a candidate to be tightly confined across four lines at once, which is an uncommon configuration, and simpler techniques usually resolve the grid before such a pattern is needed. Many puzzles that technically contain a Jellyfish can also be cracked with an X-Wing, Swordfish, or a chain, so the Jellyfish is rarely the only path forward. As a result you will encounter them far less often than smaller fish.

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