Final answer:
Vertical angles are formed when two lines intersect, and they have the same measure, making them congruent.
Step-by-step explanation:
Vertical angles are formed when two lines intersect. These angles have the same measure, which means they are congruent.
A geometric proof can be provided to show that vertical angles are congruent. By drawing a transversal line across two parallel lines, we can observe that the alternate interior angles are congruent. Since vertical angles are alternate interior angles, they must also be congruent.
For example, if angle A and angle C are vertical angles, and both angles measure 50 degrees, then angle B and angle D, which are congruent to angles A and C respectively, will also measure 50 degrees.