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 \(m\,\angle 2 = m\,\angle 4\).