Two angles are called supplementary angles of each other if the sum of their measures is equal to the straight angle , i.e. .

For example, when two lines intersect each other, they divide the plane into four disjoint domains corresponding to four convex angles; then any of these angles has a supplementary angle on either side of it (see linear pair). However, two angles that are supplementary to each other do not need to have a common side -- see e.g. an entry regarding opposing angles in a cyclic quadrilateral.