Jellyfish Sudoku: The Advanced Fish Pattern Beyond Swordfish
If you have mastered Swordfish and are looking for the next step up in the fish-pattern family, Jellyfish is it. Rare but decisive, a Jellyfish eliminates candidates across four rows and four columns at once — and once you understand how it scales from the smaller fish, spotting it becomes systematic rather than mysterious.
What Is the Jellyfish Pattern?
A Jellyfish is a four-row (or four-column) fish pattern. The rule is:
If a candidate digit appears in exactly 2, 3, or 4 cells across each of four rows, and all of those candidate cells collectively fall within the same four columns, then that digit must be placed in those rows within those four columns. Every other occurrence of that candidate in those same four columns — but outside the four base rows — can be eliminated.
The logic is a direct extension of X-Wing (2 rows, 2 columns) and Swordfish (3 rows, 3 columns). Jellyfish adds one more row and one more column to the fish. For a complete breakdown of the rules, see the Jellyfish strategy page.
Jellyfish vs Swordfish — How It Scales
Understanding the fish family as a progression makes each pattern easier to learn:
- X-Wing: 2 base rows, candidate confined to 2 columns. Eliminates from those 2 columns outside the 2 rows.
- Swordfish: 3 base rows, candidate confined to 3 columns. Eliminates from those 3 columns outside the 3 rows.
- Jellyfish: 4 base rows, candidate confined to 4 columns. Eliminates from those 4 columns outside the 4 rows.
Each step up adds one row and one column to the constraint set. The underlying logic is identical at every level: if N rows between them can only place a candidate in N columns, those N columns are fully "used up" by those rows, so any other cell in those columns cannot hold that candidate.
Jellyfish is rarer than Swordfish in standard puzzles because the constraint is harder to achieve — four rows must simultaneously confine their candidates to the same four columns. On Expert and Master grids, however, it does appear, and recognizing it immediately is a significant skill advantage.
How to Spot Jellyfish (4-Row/Column Check)
The systematic scan mirrors Swordfish, extended to four rows:
- Pick a candidate digit. Scan your pencil marks for one digit to focus on.
- List the rows where that digit has 2, 3, or 4 candidate cells. Skip any row where the digit appears in 5 or more cells — it cannot be part of a Jellyfish base.
- Choose 4 of those rows. Check whether all candidate cells across those 4 rows fall within the same set of 4 columns.
- Confirm the column count. Count the distinct columns used. If the total is exactly 4 (not 3, not 5), you have a Jellyfish.
- Eliminate. Remove that candidate from every other cell in those 4 columns — any cell that is not in one of the 4 base rows.
A useful shortcut: Jellyfish works symmetrically. You can start with 4 columns and look for 4 rows just as easily. If your column-based scan is productive, use it. Whichever direction you scan first, the elimination runs in the opposite direction.
Finned Jellyfish: When the Pattern Has Extra Candidates
A finned Jellyfish occurs when the base pattern almost holds — but one of the four rows has a candidate cell that falls outside the four defining columns. That extra cell is called the fin.
The fin breaks the strict Jellyfish rule, which means you cannot apply the full set of eliminations. However, the fin does allow a restricted elimination: any candidate cell that is both in the cover columns and in the same 3×3 box as the fin can be eliminated. Cells outside that box's row-set cannot be eliminated.
In practice, finned Jellyfish produce fewer eliminations than a clean Jellyfish, but they are far more common. If you find four rows that almost form a Jellyfish and one row has a stray extra candidate, check whether it is a finned variant before dismissing it. For the full rules and examples see the Finned Jellyfish strategy page.
Step-by-Step Example
Consider this setup for the digit 7:
- Row 1: candidate 7 in columns 2 and 5
- Row 3: candidate 7 in columns 2, 5, and 8
- Row 6: candidate 7 in columns 5 and 8
- Row 8: candidate 7 in columns 2 and 8
The four base rows are 1, 3, 6, and 8. The candidate cells collectively land in columns 2, 5, and 8 — only three distinct columns. That makes this a Swordfish, not a Jellyfish. Now suppose Row 3 also has a candidate 7 in column 4:
- Row 3 (revised): candidate 7 in columns 2, 4, 5, and 8
Now the four rows use columns 2, 4, 5, and 8 — exactly four. That satisfies the Jellyfish definition.
Elimination: Remove candidate 7 from every other cell in columns 2, 4, 5, and 8 that is not in rows 1, 3, 6, or 8. If row 5, column 5 had a pencil-mark 7, cross it out. If row 9, column 2 had a 7, cross that out too. Work through every cell in those four columns that sits outside the four base rows.
After clearing those candidates, check whether any newly isolated single has emerged in the affected cells — a Jellyfish elimination often sets off a chain of simpler moves.
When Will You Need Jellyfish?
Jellyfish is an Expert and Master difficulty technique. You will almost never encounter it on Easy, Medium, or Hard puzzles — those grids are designed to be solvable with techniques up to and including Swordfish, hidden pairs, and basic chains.
On Expert and Master grids where singles, locked candidates, naked and hidden subsets, and even Swordfish have all been exhausted, Jellyfish is worth scanning for. Because it is rare, the investment is low: a thorough candidate-list review for each digit in four-row combinations takes only a minute once you have practiced the pattern.
A key sign that a Jellyfish might be present: after applying all simpler techniques, one digit stubbornly persists as a candidate across many cells in several rows and columns, with no clear placement path. That pattern of distributed candidates is exactly the setup where fish techniques — including Jellyfish — tend to appear.
Want to practice Expert-level solving? Try today's daily Sudoku and work your way up through the fish family.