Two angles are supplementary angles when the sum of their measures is $180^{\circ }.$ In the diagram, $\angle B$ and $\angle E$ are supplementary because the sum of their measures equals $180^{\circ }.$ When using radians as the unit of measure, two angles are supplementary when the sum of their measures is $\pi ,$ or about $3.14,$ radians. If two angles are supplementary, either both are right angles or one is acute and the other obtuse. When two supplementary angles are adjacent, they are called a linear pair or straight angle pair. Notice that a linear pair forms a straight angle.