Jellyfish is one of the most complex fish-pattern techniques in Sudoku, extending the logic of X-Wing and Swordfish to encompass four rows and four columns simultaneously. Just as an X-Wing uses two rows and two columns and a Swordfish uses three of each, a Jellyfish pattern involves a single candidate digit that appears in exactly four rows, with all occurrences confined to the same four columns (or vice versa). When this pattern is identified, that candidate can be eliminated from all other cells in those four columns.
Identifying a Jellyfish requires meticulous scanning of candidate positions across the entire grid. For a given digit, examine each row and note which columns contain that digit as a candidate. If you can find four rows where the candidate appears only within the same set of four columns, you have found a Jellyfish. The candidate must appear in at least two of the four columns in each row, though not every row needs to use all four columns. The elimination then removes that candidate from all other cells in those four columns that are not part of the defining four rows.
The Jellyfish technique is rare in standard puzzles but becomes essential when tackling fiendishly difficult Sudoku grids. It is the natural progression from X-Wing (2x2) through Swordfish (3x3) to this 4x4 pattern, and understanding all three as a family of techniques helps solidify your grasp of set-based candidate elimination. Practicing Jellyfish identification strengthens your ability to track candidate distributions across the grid and prepares you for even more advanced chain-based solving strategies.
Try It Yourself
Walk through each step of the jellyfish technique on a real puzzle. Follow the instructions and try entering the correct value when prompted.
We are tracking candidate 2 across the grid. Examine rows 1, 3, 5, and 7 for the positions where 2 can appear. Note which columns contain candidate 2 in each of these rows.
Step-by-Step Guide
Choose a candidate digit to analyze and note its positions in every row of the grid.
Find rows where the chosen digit appears as a candidate in only a limited number of columns.
Attempt to group exactly four rows where the candidate is confined to the same four columns.
Verify that in each of the four rows, the candidate only appears within those four columns.
Confirm that not all occurrences can be explained by a smaller fish pattern (X-Wing or Swordfish).
Eliminate the candidate from all other cells in the four columns that are NOT in the four Jellyfish rows.
Re-evaluate affected cells — the eliminations may reveal naked singles or enable further techniques.
If no row-based Jellyfish is found, repeat the analysis using columns as the base and rows as the cover.
Think of it like a four-way intersection where the same car can only be parked in certain spots across four streets -- once you know the cars must fill those spots, no other car of that type can park anywhere else on those streets.
The Jellyfish works because of the generalized cover set principle: if a digit's occurrences in four base rows are confined to at most four columns, then those four rows must collectively consume all available positions for that digit within those four columns. Since each row must place the digit exactly once and can only do so within the cover columns, the four placements fully saturate the four columns, leaving no room for the digit in any other row within those columns.
When to use: Use Jellyfish when you have exhausted X-Wing and Swordfish searches and a candidate digit still appears in a restrictive pattern across exactly four rows or four columns.
Common Mistakes to Avoid
Including five or more rows/columns instead of exactly four, which is not a valid Jellyfish pattern.
Double-check that the candidate is confined to exactly four rows and four columns. If it spans more, the pattern does not qualify as a Jellyfish.
Eliminating candidates from the defining rows instead of the covering columns (or vice versa).
Eliminations always happen in the cover sets. If your base sets are rows, eliminate from the four columns outside the base rows, not from within the rows themselves.
Confusing a Jellyfish with a Swordfish when one of the four rows only has candidates in two columns.
A row with candidates in only two of the four columns is still valid in a Jellyfish. Not every row needs to use all four columns -- just verify that all candidates fall within those four columns.
More Examples
See jellyfish applied in different puzzle configurations to strengthen your pattern recognition.
Jellyfish on four rows and columns
Highlighted cells show the jellyfish pattern
Practice Puzzles
Apply the jellyfish technique on these mini challenges. Tap a highlighted cell and enter the correct digit.
Quick Reference
- Pattern:
- A candidate in 2-4 cells per row across four rows, confined to four columns
- Action:
- Eliminate that candidate from all other cells in those four columns
- Look for:
- A candidate forming a 4x4 fish pattern across rows and columns