Illustrated · 1 min
Vertical angles are across from each other at a vertex
When two lines intersect, they form four angles. The two angles that are directly across the intersection point from each other are called vertical angles. Vertical angles are always congruent.
In the image above, the two angles marked in blue (DXA and BXC) are vertical to each other and thus congruent. The angles marked in green (AXC and DXB) are also vertical to each other and congruent.