X-Wing

An X-Wing uses a digit that appears in exactly two cells in each of two rows or columns. The four cells form a rectangle.

In the example above 5 appears in exactly two cells in columns 2 and 6, and the candidates line up in rows 2 and 4. The four cells form an X-Wing. Remove 5 from the other cells in rows 2 and 4 (red).

You can also form an X-Wing the other way around: two rows with candidates in the same two columns. In that case, remove the digit from those columns.