What is Swordfish in Sudoku?

Swordfish is an advanced technique like X-Wing but extended to three rows and three columns. A digit appears as a candidate in two or three cells in each of three rows, and all those cells are confined to the same three columns. This allows eliminating the digit from those columns in all other rows.

How is Swordfish different from X-Wing?

X-Wing involves two rows and two columns forming a rectangle. Swordfish involves three rows and three columns — the pattern is more complex and harder to spot, but the logic is the same: the digit is locked into a rectangular arrangement that allows column-wide eliminations.

Swordfish | Sudoku Solving Technique Guide

What is the Swordfish Technique?

The Swordfish is an advanced Sudoku solving technique that gets its name from the swordfish-like pattern formed by six or nine cells arranged across three rows and three columns. It's a powerful elimination method that can help you solve extremely difficult puzzles.

This technique is the natural extension of the X-Wing pattern, which uses two rows and two columns. The Swordfish uses three rows and three columns, making it more complex but also more powerful for eliminating candidates.

Understanding the Swordfish Pattern

A Swordfish occurs when:

1A specific digit appears in exactly two or three cells in each of three rows
2These cells are aligned in exactly three columns
3The digit can be eliminated from other cells in those three columns

The same logic applies when looking at columns instead of rows - a digit appears in exactly two or three cells in each of three columns, aligned in three rows.

Why Does the Swordfish Work?

The Swordfish works because of the fundamental Sudoku rules. If a digit can only appear in two or three specific cells in each of three rows, and these cells are aligned in columns, then:

•In each row, the digit must go in one of its possible cells
•Since the cells are aligned in three columns, the digit must occupy all three columns
•Therefore, the digit cannot appear anywhere else in those columns

Key Insight

The Swordfish pattern ensures that a digit must appear in each of the three columns involved. This is because each row must contain the digit exactly once, and the alignment forces the digit to be placed in the three specific columns.

Step-by-Step Example

Let's work through a detailed example. In the puzzle below, we'll look for a Swordfish pattern involving the digit 8:

The highlighted cells form a Swordfish pattern for the digit 8 across rows 1, 3, and 5, aligned in columns 3 and 6.

Step 1: Check Row 1 (Index 0)

In row 1, the digit 8 can only appear in cells (1,3) and (1,6). These are the only empty cells in this row where 8 is a possible candidate.

Step 2: Check Row 3 (Index 2)

In row 3, the digit 8 can only appear in cells (3,3) and (3,6). Again, these are the only empty cells in this row where 8 is a possible candidate.

Step 3: Check Row 5 (Index 4)

In row 5, the digit 8 can only appear in cells (5,3) and (5,6). These are the only empty cells in this row where 8 is a possible candidate.

Step 4: Verify Column Alignment

The six cells form a perfect Swordfish pattern:

• Row 1: cells (1,3) and (1,6)
• Row 3: cells (3,3) and (3,6)
• Row 5: cells (5,3) and (5,6)
• Columns: 3 and 6

Step 5: Apply the Elimination

Since 8 must appear in both columns 3 and 6 (one in each column), we can eliminate 8 from all other cells in these columns.

How to Spot Swordfish Patterns

Method 1: Row-Based Search

1.Look for rows where a specific digit appears in exactly 2-3 cells
2.Find at least three such rows
3.Check if the cells align in exactly three columns
4.If found, you have a Swordfish pattern

Method 2: Column-Based Search

1.Look for columns where a specific digit appears in exactly 2-3 cells
2.Find at least three such columns
3.Check if the cells align in exactly three rows
4.If found, you have a Swordfish pattern

Method 3: Systematic Approach

1.Choose a digit (1-9) to focus on
2.Mark all possible positions for that digit
3.Look for three rows/columns with 2-3 candidates each
4.Verify proper alignment in three columns/rows

Practice Exercise

Try to find Swordfish patterns in this practice puzzle. Look for digits that appear in exactly two or three cells in each of three rows or columns:

Look for Swordfish patterns in this puzzle. Focus on digits that appear in 2-3 cells in each of three rows or columns.

Common Swordfish Variations

1. Standard Swordfish (6 cells)

The classic Swordfish with exactly two cells per row, forming a 3×2 pattern across three rows and two columns.

2. Extended Swordfish (9 cells)

A Swordfish with exactly three cells per row, forming a 3×3 pattern across three rows and three columns.

3. Mixed Swordfish

A Swordfish where some rows have two candidates and others have three, but still aligned in three columns.

4. Multiple Swordfish

A puzzle might contain multiple Swordfish patterns for different digits, or overlapping Swordfish patterns that can be applied in sequence.

Tips for Success

Best Practices

•Use pencil marks: Keep detailed track of all possible candidates
•Look for patterns: Swordfish often appear in areas with many constraints
•Check alignment carefully: Verify that cells are properly aligned in three columns/rows
•Practice regularly: The more you look for Swordfish, the easier they become to spot

Common Mistakes to Avoid

•Incorrect alignment: Make sure the cells form a proper 3×3 or 3×2 pattern
•Wrong count: The digit must appear in exactly 2-3 cells in each row/column
•Missing eliminations: Don't forget to eliminate from other cells in the affected columns/rows
•Overlooking variations: Check both row-based and column-based Swordfish

When to Use Swordfish

Swordfish is most effective when:

•You have excellent pencil marks showing possible candidates
•Basic and intermediate techniques are no longer sufficient
•You're working on expert or master-level puzzles
•X-Wing patterns are not applicable but similar patterns exist

Advanced Applications

Once you master Swordfish, you can explore even more advanced techniques:

•Jellyfish: An extension of Swordfish involving four rows and four columns
•Finned Swordfish: A Swordfish pattern with additional candidates that can be handled with special logic
•Remote Pairs: Complex patterns that span across multiple units

Next Steps

The Swordfish technique is one of the most powerful advanced patterns in Sudoku. Once you're comfortable with Swordfish, consider learning about Note Validation (automated pencil mark management) or Bruteforce (last-resort solving method).

Ready for More Advanced Techniques?

Practice your Swordfish skills or learn the next advanced technique.