Swordfish Technique

What is the Swordfish Technique?

The Swordfish is an advanced fish pattern that operates on the same principle as the X-Wing, but involves three rows and three columns instead of two. Like all fish patterns, it creates a rectangular constraint that allows for eliminations.

A Swordfish occurs when a candidate number appears in exactly 2-3 cells in each of three rows (or columns), and these candidates align to form three columns (or rows) that intersect all three rows. This creates a pattern where the number must appear exactly once in each of the three rows and three columns involved.

Key insight: Since each row must contain the number exactly once, and the candidates are confined to three specific columns, all other instances of that candidate in those three columns can be eliminated.

How Swordfish Works

Requirements

1 Find a candidate that appears in 2-3 cells in exactly three rows
2 These candidates must align to form three columns
3 The pattern must be consistent across all three rows

Result

All other instances of the candidate in the three columns (outside the three rows) can be eliminated.

This often creates chain reactions that lead to solving multiple cells or revealing other patterns.

Step-by-Step Example

Before: Identifying the Swordfish Pattern

Let's look for the number 7 and see if we can find a Swordfish pattern. We need to examine rows 2, 5, and 8 where the candidate 7 appears in specific positions.

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Swordfish Pattern Identified: The number 7 appears in columns 2, 4, and 7 across rows 2, 5, and 8. This forms a Swordfish pattern highlighted in blue.

After: Eliminations Made

Since the Swordfish pattern constrains the number 7 to specific positions in rows 2, 5, and 8, we can eliminate all other 7s in columns 2, 4, and 7 (shown in red).

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Eliminations: All 7s in columns 2, 4, and 7 that are outside the Swordfish pattern (rows 2, 5, and 8) have been eliminated, shown with strikethrough in red cells.

How to Find Swordfish Patterns

Systematic Approach

1

Choose a candidate number

Pick a number that appears to have limited placements in multiple rows or columns.

2

Examine three rows (or columns)

Look for three rows where the candidate appears in exactly 2-3 cells each.

3

Check column alignment

Verify that all candidates align to form exactly three columns across the three rows.

4

Apply eliminations

Remove all other instances of the candidate from the three involved columns.

⚠️ Common Mistakes

• Overlooking incomplete patterns where candidates don't align properly across all three rows
• Confusing Swordfish with simpler patterns like X-Wing or multiple Pointing Pairs
• Eliminating candidates from the wrong cells (remember: eliminate from columns, not rows)
• Missing the symmetric version where rows and columns are swapped

Practice Exercises

💡 Practice Tips

• Start by looking for simpler fish patterns (X-Wing) before attempting Swordfish
• Use pencil marks extensively to track candidate placements
• Focus on candidates that appear limited in multiple regions
• Practice with expert-level puzzles where advanced techniques are required
• Try online Sudoku solvers that highlight fish patterns for learning