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

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

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