1. What is a Sudoku Swordfish?

A Swordfish pattern occurs when a candidate number (for example, the digit 5) appears in 2 or 3 cells in each of three separate rows, and all of these candidate cells are confined to the exact same three columns.

Geometric Swordfish Grid (Digit 5 across Rows 1, 6, 9):

        Col 2         Col 5         Col 8
Row 1:  [ 5 ] --------- [ 5 ] --------- [ ]   (Row 1 candidate 5 only in C2, C5)
          |               |               |
Row 6:  [ ] --------- [ 5 ] --------- [ 5 ]   (Row 6 candidate 5 only in C5, C8)
          |               |               |
Row 9:  [ 5 ] --------- [ ] --------- [ 5 ]   (Row 9 candidate 5 only in C2, C8)

Notice that not every row needs to have all three candidate cells! Each row only needs 2 or 3 cells, as long as the entire collection of cells across all three rows is strictly contained within Columns 2, 5, and 8.

2. The Elimination Action (The Swordfish Payoff)

Because each of the three rows must contain one instance of digit 5, and the candidate cells are limited to Columns 2, 5, and 8:

Golden Rule of Swordfish:

Digit 5 is guaranteed to occupy one cell in each of the three columns (Col 2, Col 5, Col 8). Therefore, candidate 5 CANNOT appear anywhere else in Columns 2, 5, and 8! You can safely erase candidate 5 from all other rows in those three columns.

3. How to Spot a Swordfish (4-Step Mental Routine)

Select a single candidate digit: Highlight one number at a time (e.g. tap digit 5 in iLoveSudoku's keypad to highlight candidate notes).
Find rows with 2 or 3 candidates: Scan rows that only have 2 or 3 cells containing that candidate note. Ignore rows with 4 or more candidates.
Check for column alignment: Test if three of those qualifying rows share the exact same 3 columns. If they do, you have discovered a Swordfish!
Execute eliminations: Erase the candidate from all other rows in those three shared columns. Often, this reveals an instant Naked Single!

4. The Fish Family Hierarchy

2x2 Fish (X-Wing)

2 rows and 2 columns. The simplest geometric rectangular elimination.

3x3 Fish (Swordfish)

3 rows and 3 columns. The powerhouse strategy for Difficult & Diabolical boards.

4x4 Fish (Jellyfish)

4 rows and 4 columns. Rare, but mathematically identical in principle.

Put the Swordfish to the Test

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