X-Wing

The X-Wing is an advanced technique used to eliminate candidates in very hard puzzles. It requires looking at the grid globally rather than just focusing on a single block.

How it Works

An X-Wing occurs when a specific candidate number can only be placed in exactly two cells in a row, and this same pattern is repeated in another row, perfectly aligned in the exact same two columns.

Think of it as a rectangle. Because the rules of Sudoku dictate that the number must appear once in each row and once in each column, the true placements must form a diagonal across that rectangle (the "X").

Consequently, the number cannot appear anywhere else in those two vertical columns outside of our four corner cells. You can safely erase it from the pencil marks of all other cells in those columns!