Grafite Sudoku
The X-Wing in sudoku
The X-Wing is a technique for hard sudoku. It is about a single number and four cells at the corners of a rectangle.
The pattern
If in two rows a number has only two possible spots, and those spots are in the same two columns, the number takes two opposite corners of the rectangle: one diagonal or the other. Either way, in those two columns the number already sits in those rows.
So you can remove it from every other cell of the two columns.
In the example, in rows 2 and 7 the 5 can only go in columns 3 and 8. The 5 is erased from the other cells of columns 3 and 8: here from rows 4, 5 and 9.
The other way round
It also works with rows and columns swapped: if in two columns a number has only two spots, in the same two rows, remove it from the rest of those rows.
You need pencil marks on the whole grid to see it. Hunt for it one number at a time.
In the app: "X-Wing" lesson in the Sudoku Academy
In the Sudoku Academy of Grafite Sudoku this technique is an interactive lesson on the grid, with exercises from real puzzles.