Vertical angles are the two nonadjacent angles formed by two intersecting lines. They share the same
vertex, and their noncommon sides
form opposite pairs of rays. In the figure, 
and 
have the same measure 
. Each forms a pair of supplementary
angles with 
,
since the adjacent pairs are linear pairs. Therefore,
vertical angles are congruent.
The term "vertical" is sometimes explained as referring to the common vertex and sometimes as meaning vertically opposite. Vertical angles are therefore also called opposite angles.
