Swordfish Sudoku Technique Explained

The X-Wing's bigger, more powerful sibling

Swordfish example in Sudoku

What is a Swordfish?

A Swordfish is an extension of the X-Wing pattern, using three rows and three columns instead of two. When a candidate appears in 2-3 cells in each of three rows, and all those cells fall within the same three columns, powerful eliminations become possible.

The pattern gets its name from the three-pronged shape it can form when visualised on the grid.

The Swordfish Principle: If a candidate appears in 2-3 cells in each of three rows, and all those cells are confined to the same three columns, the candidate can be eliminated from all other cells in those three columns.

Swordfish vs X-Wing

X-Wing

2 rows × 2 columns

Exactly 2 cells per row

4 cells total (rectangle)

Swordfish

3 rows × 3 columns

2-3 cells per row

6-9 cells total (irregular)

The Flexibility of Swordfish

Unlike X-Wing which requires exactly 2 cells per row, Swordfish allows 2 OR 3 cells per row. This makes the pattern more flexible but also harder to spot. Valid configurations include:

How to Find Swordfish

Step-by-Step Process:

  1. Choose a candidate: Pick a number to search for
  2. Find restricted rows: Look for rows where the candidate appears in only 2-3 cells
  3. Check column alignment: Do three such rows share the same three columns?
  4. Verify the pattern: All candidate cells in these rows must fall within those three columns
  5. Make eliminations: Remove the candidate from other cells in those three columns

Why Swordfish Works

The Logic

Each of the three rows must have exactly one instance of the candidate. Since all possible positions fall within three columns, those three columns will receive exactly three instances of the candidate (one from each row).

Therefore, no other cell in those three columns can contain the candidate – they're already "claimed" by the Swordfish rows.

Example Analysis

Finding a Swordfish

Searching for candidate 4:

  • Row 2: 4 appears in columns 1, 5, 8
  • Row 5: 4 appears in columns 1, 8
  • Row 8: 4 appears in columns 5, 8

Swordfish found! All cells fall within columns 1, 5, and 8.

Elimination: Remove 4 from all other cells in columns 1, 5, and 8.

Common Challenges

  • Harder to visualise: The irregular shape makes Swordfish harder to spot than X-Wing
  • More cells to track: You're managing 6-9 cells instead of 4
  • Column alignment: All cells must fall within exactly three columns
  • Partial rows: Rows with only 2 cells are valid but easy to overlook
Pro Tip: Start by finding X-Wings, then look for a third row that could extend the pattern. If you find two rows that almost form an X-Wing but use three columns instead of two, look for a third row to complete a Swordfish.

Row-Based vs Column-Based

Like X-Wing, Swordfish can be found in two orientations:

Row-Based Swordfish

Start with 3 rows, eliminate from columns

Column-Based Swordfish

Start with 3 columns, eliminate from rows

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