What is the definition of vertical angles?

Vertical angles are angles across from each other at the intersection of two lines.

Identify the vertical angles in the diagram below. (Imagine an 'X' shape. Angle A is top left, Angle B is top right, Angle C is bottom right, Angle D is bottom left.) Which angles are vertical angles?

Angle A and Angle C; Angle B and Angle D

A transversal line is a line that intersects two or more other lines.

I can identify and solve for vertical angles formed by intersecting lines.

1
Vertical angles are angles across from each other at the intersection of two lines, and they are always equal.
2
When two parallel lines are cut by a transversal line, two intersections are formed, each containing two pairs of vertical angles.
3
To solve for unknown angles, remember that vertical angles are equal in measure, so you can set up a matching game across the intersection.