Swordfish Explained - Sudoku World Tournament

What is Swordfish?

Swordfish is an advanced Sudoku solving technique that allows you to eliminate candidates by identifying a specific pattern across three rows or three columns. It's an extension of the simpler X-Wing technique, but applied to three lines instead of two.

The name comes from the visual pattern that forms when you connect the candidate positions – it resembles a swordfish with its fins extended!

Swordfish pattern candidates (in column 4)

✓

Candidates that can be eliminated

The Swordfish Pattern

A Swordfish occurs when a candidate number appears in exactly two or three cells in each of three rows (or columns), and these cells align in exactly three columns (or rows). When this pattern exists, you can eliminate that candidate from all other cells in those three columns (or rows).

                Key Principle: If a number can only appear in three specific columns across three rows (or three specific rows across three columns), then that number MUST occupy exactly three cells within those intersections. Therefore, the number cannot appear anywhere else in those three columns (or rows).
            

Identifying a Swordfish

Step-by-Step Process:

Choose a candidate number to analyse (in our example, we're looking at candidates in column 4)
Find three rows where this candidate appears in only 2-3 cells
Check alignment – these candidates must be confined to exactly three columns
Verify the pattern – each of the three columns must contain at least two of the candidate cells from the three rows
Eliminate candidates – remove the candidate from all other cells in the three columns (outside the three defining rows)

Analysing the Example

Understanding the Image

In the displayed puzzle, column 4 shows a Swordfish pattern with several candidates marked with red circles. This visual representation helps identify where the pattern exists.

The Pattern: When examining the candidates (marked in red circles), they form a Swordfish configuration. The candidates are distributed across three rows, appearing in positions that align within three specific columns.

The Eliminations: The green tick marks (✓) indicate positions where candidates can be eliminated as a result of the Swordfish. Once you identify that certain columns must contain the candidate within the three defining rows, you can confidently remove that candidate from all other cells in those columns.

Column-Based vs Row-Based Swordfish

Column-Based Swordfish

In a column-based Swordfish, you find three columns where a candidate appears in only 2-3 cells, and these cells align within exactly three rows. You then eliminate the candidate from other cells in those three rows.

Row-Based Swordfish

In a row-based Swordfish (like the pattern shown in the example), you find three rows where a candidate appears in only 2-3 cells, and these cells align within exactly three columns. You then eliminate the candidate from other cells in those three columns.

Note: The Swordfish doesn't tell you WHERE the number goes – it only tells you where it CANNOT go. This elimination often creates simpler patterns that you can then solve using more basic techniques.

When to Use Swordfish

Look for Swordfish when:

You've exhausted simpler techniques like naked singles, hidden singles, and X-Wing
You notice a candidate appearing in a limited, structured pattern across multiple rows or columns
The puzzle is at an intermediate to advanced difficulty level
You need to break through a particularly stubborn section

Common Mistakes to Avoid

                Miscounting cells: Ensure each of the three rows (or columns) has exactly 2-3 candidates
Wrong alignment: The candidates must align in exactly three columns (or rows), not four or more
Forgetting completeness: All possible positions must be considered, not just convenient ones
Eliminating from wrong areas: Only eliminate from the three defining columns/rows, not from the three defining rows/columns

            

Practice Makes Perfect

The Swordfish technique requires practice to spot quickly. Start by:

Examining one candidate number at a time
Using pencil marks to track all candidates clearly
Drawing lines or highlighting to visualise the pattern
Checking your work carefully before making eliminations

With experience, you'll begin to recognise Swordfish patterns more quickly, making it a powerful addition to your Sudoku solving toolkit.

                Pro Tip: If you're struggling to find a Swordfish, try analysing candidates that appear frequently but not everywhere. Numbers that appear in 6-9 cells across the entire grid often form these kinds of patterns.
            