"Swordfish" Technique

What is the "Swordfish" Technique?

The "Swordfish" technique is an advanced Sudoku strategy. It's usually applied in the hard levels of Sudoku puzzles to eliminate candidates. "Swordfish" is similar to X-wing but uses three sets of cells instead of two.

How to Find a Swordfish

To understand better, let's take a look at the example.

Step 1: Find the "Fish Digit" and Base Sets

In this puzzle, 6 is our "fish digit". We must find three rows (or columns) where the candidate '6' appears only in the same three columns (or rows).

As the example shows, our base sets are Rows 1, 4, and 9. When we scan these three rows, we find that the candidate '6' only appears in Columns 1, 8, and 9.

Finding Swordfish - fish digit 6 creates 3x3 pattern for large-scale elimination

Step 2: The Logic (Two Main Possibilities)

The candidate '6' must be placed once in each of these three rows (Row 1, Row 4, Row 9). Because they all align perfectly with Columns 1, 8, and 9, there are two main "diagonal" patterns for the solution:

Case 1: The '6's might be placed in a pattern like R1C8, R4C9, R9C1.

Swordfish Case 1 - first diagonal pattern showing possible placement of candidate 6

Case 2: Or, they might be placed in a pattern like R1C9, R4C1, R9C8.

Swordfish Case 2 - alternative diagonal pattern showing different placement of candidate 6

Step 3: Eliminate Candidates

Either way, those three sets (Rows 1, 4, 9) cover the aligned columns (Columns 1, 8, 9). This means that the '6's for these rows must be in these three columns.

Therefore, no other cell in these three columns can be a '6'.

Hence, we can safely eliminate 6 from all other notes in these three columns (i.e., in Rows 2, 3, 5, 6, 7, and 8).

Swordfish result - powerful 3x3 pattern cleans up entire columns efficiently

Master This Advanced Technique

Now you know how to apply the "Swordfish" technique in Sudoku. It's very hard to spot but tremendously useful for your Sudoku-solving arsenal.