Two angles are supplementary angles when the sum of their measures is 180∘.
In the diagram, ∠B and ∠E are supplementary because the sum of their measures equals 180∘.
When using radians as the unit of measure, two angles are supplementary when the sum of their measures is π, 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.