Swordfish Sudoku Technique Explained

The Swordfish technique is an advanced Sudoku solving method that extends the concept of the X-Wing pattern to three rows and columns. It's one of the most powerful fish patterns in Sudoku and can help you solve even the most challenging puzzles.

What is the Swordfish Technique?

The Swordfish technique is a pattern recognition method that works when a specific digit appears in exactly three rows (or columns) and exactly three columns (or rows). This creates a pattern that allows you to eliminate candidates from other cells.

Think of it as an extension of the X-Wing technique:

• X-Wing: 2 rows × 2 columns
• Swordfish: 3 rows × 3 columns

How the Swordfish Pattern Works

For a Swordfish pattern to be valid, you need:

1. A specific digit that appears as a candidate in exactly three rows
2. These three rows must share exactly three columns
3. The digit must appear in exactly three cells total across these rows and columns

When this pattern exists, you can eliminate the digit as a candidate from all other cells in the three columns (or rows, depending on your perspective).

Step-by-Step Swordfish Identification

Step 1: Choose a Target Digit

Start by selecting a digit that appears frequently in your puzzle. Look for digits that have candidates in multiple rows and columns.

Step 2: Find Three Rows with the Digit

Identify three rows where your target digit appears as a candidate. The digit should appear in exactly three rows total.

Step 3: Check Column Alignment

Verify that these three rows share exactly three columns where the digit appears. The pattern should form a 3×3 grid of possible positions.

Step 4: Apply the Elimination

If the pattern is valid, eliminate the target digit from all other cells in the three columns that are not part of the Swordfish pattern.

Swordfish Example

Let's say we're looking for digit 7 in a Swordfish pattern:

This creates a Swordfish pattern where:

• Three rows (2, 5, 8) contain digit 7
• Three columns (1, 4, 7, 8) are involved
• Wait - this is actually 4 columns, not 3!

This is NOT a valid Swordfish pattern because it involves 4 columns instead of exactly 3.

Valid Swordfish Example

Here's a correct Swordfish pattern for digit 5:

This creates a valid Swordfish because:

• Exactly 3 rows (1, 4, 7) contain digit 5
• Exactly 3 columns (2, 6, 9) are involved
• Each row has exactly 2 candidates for digit 5

You can now eliminate digit 5 from all other cells in columns 2, 6, and 9 that are not in rows 1, 4, and 7.

Common Swordfish Variations

Row-Based Swordfish

When you look for the pattern across three rows, you eliminate candidates in the corresponding three columns.

Column-Based Swordfish

When you look for the pattern across three columns, you eliminate candidates in the corresponding three rows.

Finned Swordfish

A Finned Swordfish occurs when one of the three rows (or columns) has an extra candidate that breaks the perfect 3×3 pattern. This is an advanced variation that requires additional logic.

Tips for Finding Swordfish Patterns

1. Use Pencil Marks

Always use pencil marks to track all possible candidates. Swordfish patterns are impossible to spot without seeing all the candidates for each digit.

2. Look for Digits with Many Candidates

Focus on digits that appear as candidates in many cells. Digits with only a few candidates are less likely to form Swordfish patterns.

3. Check Both Rows and Columns

Remember that Swordfish can work in either direction - look for patterns in both rows and columns.

4. Practice Pattern Recognition

The more you practice, the easier it becomes to spot these patterns. Start with simpler fish patterns like X-Wing before moving to Swordfish.

When to Use Swordfish

Use the Swordfish technique when:

• Basic techniques (singles, pairs, triples) no longer provide progress
• You've already tried X-Wing and Y-Wing techniques
• You're working on expert-level puzzles
• A digit appears frequently across multiple rows and columns

Common Mistakes to Avoid

1. Incorrect Row/Column Count

Make sure you have exactly 3 rows and exactly 3 columns. Having 4 or more columns (or rows) doesn't create a valid Swordfish.

2. Missing Candidates

Ensure you've marked all possible candidates with pencil marks. Missing candidates can lead to incorrect pattern identification.

3. Wrong Elimination

Only eliminate candidates from cells that are NOT part of the Swordfish pattern. Don't eliminate from the pattern cells themselves.

Advanced Swordfish Concepts

Mutant Swordfish

A Mutant Swordfish is a variation where the pattern doesn't form a perfect 3×3 grid but still follows the same elimination logic.

Sashimi Swordfish

A Sashimi Swordfish is similar to a Finned Swordfish but with a different fin configuration. These are very advanced patterns.

Practice Exercises

To master the Swordfish technique:

1. Start with puzzles that are known to contain Swordfish patterns
2. Practice identifying the pattern before applying eliminations
3. Work through examples step by step
4. Try creating your own Swordfish examples

Related Techniques

The Swordfish technique is part of a family of fish patterns:

• X-Wing (2×2 pattern)
• Swordfish (3×3 pattern)
• Jellyfish (4×4 pattern)