X-Wing: the rectangle pattern
The X-Wing is the first of the advanced techniques. It works across two rows and two columns at once and removes a candidate you would otherwise miss. Click through a real example step by step below.
Interactive example
Click ‘Step’ to see the pattern and the elimination.
Try it: X-Wing
How the X-Wing works
Find a digit that appears as a candidate in two rows only in the same two columns. The four intersection cells form a rectangle. However the digit ends up distributed across those rows, it always occupies two opposite corners of the rectangle, that is both columns. So the digit cannot go anywhere else in those two columns and can be removed there. The same holds swapped, when a digit appears in two columns only in the same two rows.