A **linear pair** is a pair of adjacent angles formed when two lines intersect.

In the figure, $\angle 1$ and $\angle 2$ form a linear pair. So do $\angle 2$ and $\angle 3$, $\angle 3$ and $\angle 4$, and $\angle 1$ and $\angle 4$.

The two angles of a linear pair are always supplementary, which means their measures add up to $180\xb0$.