Vertical angles are the angles formed opposite each other when two lines intersect. Vertical angles are always equal.

In the picture to the left, \(a^\circ\) and \(b^\circ\) are vertical angles, so \(a^\circ = b^\circ\).

You're probably wondering why they're called "vertical" angles when they aren't vertical at all. In this case, "vertical" is a reference to the vertex where the two lines cross. Perhaps a more familiar term for vertical angles is "vertically opposite angles".