Vertical angles must be

Vertical angles must be congruent

Step-by-step explanation:

Vertical angles are any two angles whose sides form two pairs of opposite rays. We can think of these as opposite angles formed by an X. When two lines intersect to make an X, angles on opposite sides of the X are called vertical angles. These angles are equal, and here's the official theorem that tells you so. Vertical angles are congruent: If two angles are vertical angles, then they're congruent.