When two lines intersect, they form two pairs of opposite angles known as vertical angles. Vertical angles are congruent to each other which means they have the same angle measure.
In the figure below, \(\angle 1\) and \(\angle 3\) are vertical angles which means \(m\,\angle 1 = m\,\angle 3\).
Also, \(\angle 2\) and \(\angle 4\) are vertical angles which means \(m\,\angle 2 = m\,\angle 4\).
