Sudoku technique · First needed at: Expert ●●●●●○○○

X-Wing

This number forms a rectangle, which rules it out along two lines.

An example from a real puzzle

How to spot it

1. Find a number that has exactly two possible cells in each of two rows.
2. If those four cells line up in the same two columns, they form a rectangle.
3. Remove the number from every other cell of those two columns. The same works with columns and rows swapped.

Why it works

Each row puts the number in one of its two cells, and the two rows must use different columns — so both columns get their copy from the rectangle's corners.