Linear Pair

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 $180°$ .