How to Use Sudoku Swordfish Strategy to Solve Puzzles Faster

The Swordfish Sudoku strategy is a Sudoku solving technique that’s used to solve more advanced puzzles by eliminating candidates.

While this strategy is only required to solve difficult Sudoku puzzles, the strategy itself is not overly complex, if you know what you’re looking for. 

It’s actually quite similar to the X-Wing technique except the Swordfish strategy looks at three sets of cells instead of just two. 

So what is a Swordfish in Sudoku and how can you use it to help solve puzzles?

What is a Swordfish in Sudoku? 

A Swordfish in Sudoku is a particular pattern of cells in an arrangement where the same candidate is found in three different rows which align to form three columns (or vice versa). 

While this might sound complicated, it helps to visualize the Swordfish pattern. So let’s consider the Sudoku Swordfish example below where all the 3 candidates have been penciled in. 

While it might not immediately seem like you can eliminate any of these candidates and place a 3 anywhere in the grid, there’s a Swordfish pattern among them. 

Remember, a Sudoku Swordfish pattern involves an arrangement where the same candidate (in this case 3) is found in the three columns (or rows) that connect to form three rows (or columns). 

This Swordfish pattern in this example is highlighted below along with the rows and columns the pattern forms.

While it might not look much like a swordfish, this is what’s known as a Swordfish pattern in Sudoku. 

How to use the Sudoku Swordfish technique to help solve puzzles

You may be looking at the highlighted cells in the example above and thinking “so what?”.

But there’s actually some clever logic you can apply to eliminate some of the 3 candidates from the grid. 

In fact, you can eliminate every 3 candidate along each of the rows and columns that are not in the highlighted cells. And this is not just between the highlighted cells where you can eliminate the candidates, but the entire length of the six nine-cell rows and columns.

This means you can immediately erase three pencil-marked 3s from the grid. 

It’s important to note that you cannot eliminate any candidates just because they share the same 3×3 box. You can only eliminate them if they share the same row and/or column that makes up the Swordfish pattern.

After eliminating the three candidates, as shown above, it’s now possible to place an extra digit in the grid. 

This is because the only other candidate along with the lower-right 3 that’s been eliminated is 2. All other values either appear in the row, column, or 3×3 box except for 2 and 3. And now that 3 has been removed as a candidate, it forces the 2. 

This has a cascade effect across that grid that will lead to solving the Sudoku puzzle. All from eliminating just three candidates with the help of the Swordfish technique.