**A linear pair of angles is always supplementary.** This means that the sum of the angles of a linear pair is always 180 degrees. This is called the linear pair theorem.

The linear pair theorem is widely used in geometry. A linear pair of angles is formed when two adjacent angles are formed by two intersecting lines. In such a case, all adjacent angles form a linear pair. A pair of adjacent angles has a common vertex and a common arm. At the point of intersection of the two lines, four linear pairs of angles are formed. The linear pair theorem is widely used in calculating the measures of angles formed when two or more lines intersect each other.