The "Peixe-espada" is an advanced Sudoku strategy, typically used in difficult levels of Sudoku puzzles to eliminate candidates. The "Peixe-espada" method is similar to the "X-Wing," but it uses three sets of cells instead of two.
To understand it better, let's look at an example.
6 is our "fish digit," and rows 1, 6, and 9 are the base sets. The candidate cells for the number 6 also align perfectly in 3 columns. Because of this, there are two possible options for where the number 6 can be placed.
Either here:
Or here:
Regardless of which placement is correct, the three base sets (the rows) cover the three columns, meaning the number 6 cannot appear twice in any of them. Portanto, we can safely eliminate the candidate 6 from all other notas in these columns.
Now you know how to apply the "Peixe-espada" method in Sudoku. It can be difficult to spot, but it is a very useful addition to your Sudoku-solving arsenal.