When in space two st. lines form a right angle, they are said to be perpendicular to each other. Look at the cube **ABCDA**_{1}B_{1}C_{1}D_{1}. The skew st. lines **A**_{1}D_{1} and **CD** are perpendicular to each other. Indeed, **A**_{1}D_{1} is perpendicular to** C**_{1}D_{1}, and **C**_{1}D_{1} is parallel to **CD**.

Here are some other pairs of the perpendicular skew st. lines: **A**_{1}D_{1} and **AB**, **A**_{1}B_{1} and **BC**, **A**_{1}B_{1} and **AD**, **B**_{1}C_{1} and **AB**.

(Definition 2)