Sudoku Techniques: X-Wing
A technique for advanced puzzles that uses four cells to eliminate candidates.
What is an X-Wing?
Take a number N. Suppose that in two rows, the cells where N can go are in exactly the same two columns. Those four cells form the corners of a rectangle.
Why you can eliminate
In each of the two rows, N must go in one of those two columns, one per column. So the N for each of those two columns is taken by these two rows, and you can remove N from the other cells in those two columns. It works the same way with rows and columns swapped.
How to spot it
Pick one number and look for rows where N has only two candidate cells. If two of those rows have their candidates in the same columns, you have an X-Wing.
Practice
The Expert level on this site collects puzzles that will not budge without techniques like this one.
More articles: Sudoku Rules and How to Get Started / The Two Basic Sudoku Techniques: Singles (One Number per Cell, One Cell per Number) / How to Use Notes (Candidates) in Sudoku / Sudoku Techniques: Pairs (Naked Pair and Hidden Pair) / Sudoku Techniques: Boxes and Lines (Pointing Pairs) / Sudoku Techniques: Trial and Error (for Demon, Hell and Abyss) / How Kids Can Play Sudoku (6×6)
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