Jellyfish in Sudoku: Strategy & Technique Explained

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Published April 15, 2026 · 12 min read

If you’ve mastered the X-Wing (2×2) and Swordfish (3×3), you’re ready for the largest practical fish pattern: the Jellyfish. Where a Swordfish locks a candidate across three rows and three columns, the Jellyfish extends that logic to four rows and four columns — covering an even wider area in a single elimination step.

The Jellyfish is one of the rarest standard techniques you’ll encounter. It appears almost exclusively in expert-level puzzles, and spotting one requires careful scanning of candidate positions across the entire grid. But when you find it, the payoff is immediate and satisfying.

In this guide we’ll explain exactly what a Jellyfish is, walk through the underlying logic, and demonstrate the technique on a real puzzle with before-and-after diagrams.

✅ What Is a Jellyfish in Sudoku?

A Jellyfish occurs when a candidate digit appears in only 2 to 4 cells in each of four different rows, and all of those cells fall within the same four columns. Because the digit must go in one of these cells in each row, the four rows “lock” the digit into those four columns — and it can be eliminated from every other cell in those four columns.

The Jellyfish is the largest commonly used member of the “fish” family of techniques. It follows the same principle as the X-Wing (2×2) and the Swordfish (3×3), simply scaled up to four rows and four columns.

🧠 How It Works (The Logic)

Think of the Jellyfish as four overlapping X-Wings. Here’s the core logic:

1. In Row 1, candidate 2 can only go in columns 4, 7, and 8.
2. In Row 4, candidate 2 can only go in columns 3 and 7.
3. In Row 6, candidate 2 can only go in columns 3 and 8.
4. In Row 9, candidate 2 can only go in columns 7 and 8.

Four rows, all restricted to the same four columns: 3, 4, 7, and 8. The digit 2 must land in exactly one cell per row, and those four placements will each occupy one of the four columns. No matter how the digit is distributed across the rows, every column gets exactly one 2.

Therefore, every other cell in columns 3, 4, 7, and 8 (outside these four rows) cannot contain 2.

🔎 Step-by-Step Example

Let’s walk through a real Jellyfish. We’re looking at digit 2 across rows 1, 4, 6, and 9 in columns 3, 4, 7, and 8.

Step 1: Identify the pattern rows

Check where candidate 2 appears in each row:

• Row 1 — candidate 2 in R1C4 {2,3}, R1C7 {2,3,6,7}, and R1C8 {2,3,6,7}
• Row 4 — candidate 2 in R4C3 {2,7} and R4C7 {2,7}
• Row 6 — candidate 2 in R6C3 {2,7} and R6C8 {2,7,8}
• Row 9 — candidate 2 in R9C7 {2,6} and R9C8 {2,6}

All candidate-2 positions are confined to columns 3, 4, 7, and 8 — exactly four columns.

Step 2: Confirm the Jellyfish

Four rows, four columns, candidate 2 restricted to 2–3 cells per row, all within the same column set. This is a valid Jellyfish pattern.

Step 3: Eliminate from the columns

Remove candidate 2 from every cell in columns 3, 4, 7, and 8 that is not in rows 1, 4, 6, or 9:

• R3C8 — {2,3,4} → {3,4}
• R8C4 — {2,3} → {3}
• R8C8 — {2,3,4,7} → {3,4,7}

That’s 3 eliminations from a single Jellyfish — clearing candidate 2 from cells outside the pattern rows in columns 4 and 8. Notice that R8C4 is reduced to a naked single — it must be 3.

Step 4: Continue solving

With fewer candidates in those columns, new singles and patterns may emerge. In this case, R8C4 becomes a naked single (3), which can trigger a chain of further placements.

🕵️ How to Find a Jellyfish

🔄 Jellyfish vs Swordfish vs X-Wing

The Jellyfish is the natural extension of the Swordfish and X-Wing. They form the “fish” family and use identical logic at different scales.

Once you can spot Swordfish confidently, the leap to Jellyfish is mostly about scanning four rows at a time instead of three. The underlying principle — restricted positions locking a candidate into specific columns — is exactly the same.

📌 Row-Based vs Column-Based Jellyfish

A Jellyfish can use either rows or columns as its base. The logic is identical — only the orientation changes.

• Row-based Jellyfish: Four base rows with the candidate confined to four columns. Eliminate from the rest of those four columns. This is the type we saw in our example.
• Column-based Jellyfish: Four base columns with the candidate confined to four rows. Eliminate from the rest of those four rows.

⚠️ Common Mistakes to Avoid

1. Pattern spanning more than 4 columns

If the candidate positions across four rows span five or more columns, it’s not a Jellyfish. Verify that the union of all column positions equals exactly four before applying any eliminations.

2. Eliminating inside the pattern rows

Only eliminate the candidate from cells in the four Jellyfish columns that are outside the four base rows. Never remove candidates from cells within the pattern rows themselves.

3. Forgetting the second orientation

If no row-based Jellyfish exists, remember to check column-based patterns as well. Checking only one orientation means missing half the possible Jellyfish patterns.

4. Confusing fish sizes

An X-Wing uses 2 rows and 2 columns; a Swordfish uses 3; a Jellyfish uses 4. Make sure you have exactly four rows with positions spanning exactly four columns — not three (Swordfish) or five+.

📅 When to Look for Jellyfish

1. Basic techniques: Naked singles, hidden singles, full house.
2. Intermediate techniques: Pointing Pairs, Box/Line Reduction, Naked Pairs, Hidden Pairs.
3. Advanced techniques: X-Wing, Swordfish, XY-Wing.
4. Expert techniques: Jellyfish, XYZ-Wing, W-Wing, Chains, ALS.

Jellyfish sits at the expert tier — the final stage of the fish family. Try X-Wing and Swordfish first; only look for Jellyfish when those and other advanced techniques have been exhausted. In practice, Jellyfish appears exclusively in the hardest puzzles.

🚀 Beyond Jellyfish

The Jellyfish completes the standard fish family from X-Wing through Swordfish. While larger fish (Squirmbag, Whale, Leviathan) exist in theory, they are computationally equivalent to other techniques and virtually never needed in hand-solving. Mastering up to Jellyfish gives you a complete fish toolkit.

🎯 Practise Jellyfish

• Fill in all pencil marks: Jellyfish requires complete candidate information.
• Focus on digits with few candidates: If a digit appears in only 2–4 cells per row across multiple rows, investigate further.
• Build up from smaller fish: Look for X-Wings and Swordfish first. A Jellyfish often appears when a fourth row extends a near-Swordfish pattern.
• Verify with the solver: Use our Sudoku solver to confirm your findings.