Swordfish in Sudoku: Strategy & Technique Explained

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Published April 15, 2026 · 12 min read

If you’ve mastered the X-Wing technique, you’re ready for its bigger sibling: the Swordfish. Where an X-Wing locks a candidate across two rows and two columns, the Swordfish extends that same logic to three rows and three columns — producing even more eliminations in a single step.

The Swordfish is one of the most satisfying advanced techniques to spot in a Sudoku puzzle. It takes practice to see, but once you understand the pattern, it becomes a powerful weapon against hard and expert-level grids.

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

✅ What Is a Swordfish in Sudoku?

A Swordfish occurs when a candidate digit appears in only 2 or 3 cells in each of three different rows, and all of those cells fall within the same three columns. Because the digit must go in one of these cells in each row, the three rows “lock” the digit into those three columns — and it can be eliminated from every other cell in those three columns.

The name comes from the fact that the pattern, when visualised with connecting lines, resembles a three-pronged fish shape. It belongs to the “fish” family of techniques alongside X-Wing (2×2) and Jellyfish (4×4).

🧠 How It Works (The Logic)

Think of the Swordfish as three overlapping X-Wings. Here’s the core logic:

1. In Row 6, candidate 7 can only go in columns 1 and 3.
2. In Row 8, candidate 7 can only go in columns 3 and 5.
3. In Row 9, candidate 7 can only go in columns 1 and 5.

Three rows, all restricted to the same three columns: 1, 3, and 5. The digit 7 must land in exactly one cell per row, and those three placements will each occupy one of the three columns. No matter how the digit is distributed across the rows, every column gets exactly one 7.

Therefore, every other cell in columns 1, 3, and 5 (outside these three rows) cannot contain 7.

🔎 Step-by-Step Example

Let’s walk through a real Swordfish. We’re looking at digit 7 across rows 6, 8, and 9 in columns 1, 3, and 5.

Step 1: Identify the pattern rows

Check where candidate 7 appears in each row:

• Row 6 — candidate 7 in R6C1 {3,6,7} and R6C3 {3,5,7}
• Row 8 — candidate 7 in R8C3 {3,5,7} and R8C5 {5,6,7}
• Row 9 — candidate 7 in R9C1 {2,3,6,7} and R9C5 {6,7}

All candidate-7 positions are confined to columns 1, 3, and 5 — exactly three columns.

Step 2: Confirm the Swordfish

Three rows, three columns, candidate 7 restricted to 2 cells per row, all within the same column set. This is a valid Swordfish pattern.

Step 3: Eliminate from the columns

Remove candidate 7 from every cell in columns 1, 3, and 5 that is not in rows 6, 8, or 9:

• R4C1 — {2,3,6,7} → {2,3,6}
• R4C3 — {2,3,5,7,9} → {2,3,5,9}
• R7C1 — {2,3,6,7} → {2,3,6}
• R7C3 — {2,3,5,7} → {2,3,5}

That’s 4 eliminations from a single Swordfish — clearing candidate 7 from cells outside the pattern rows in columns 1 and 3.

Step 4: Continue solving

With fewer candidates in those columns, new singles, pairs, or other patterns may emerge. In this case, R7C3 is reduced to just {2,3,5}, potentially enabling further progress.

🕵️ How to Find a Swordfish

🔄 Swordfish vs X-Wing

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

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

📌 Row-Based vs Column-Based Swordfish

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

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

⚠️ Common Mistakes to Avoid

1. Pattern spanning more than 3 columns

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

2. Eliminating inside the pattern rows

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

3. Forgetting the second orientation

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

4. Confusing with X-Wing

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

📅 When to Look for Swordfish

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.

Swordfish sits in the advanced tier. Try X-Wing first — only look for Swordfish when X-Wing and other simpler techniques have been exhausted. In practice, Swordfish appears primarily in hard and expert-level puzzles.

🚀 Beyond Swordfish

Master the Swordfish and you’ll have unlocked the full fish family from X-Wing to Jellyfish — a progression that applies the same logical principle at increasing scale.

🎯 Practise Swordfish

• Fill in all pencil marks: Swordfish requires complete candidate information.
• Focus on digits with few candidates: If a digit appears in only 2–3 cells per row across multiple rows, investigate further.
• Check both orientations: Test rows first, then columns.
• Verify with the solver: Use our Sudoku solver to confirm your findings.