Which of the following statements is true about vertical angles?

Vertical angles are formed when two lines intersect, creating two pairs of opposite angles. The key characteristic of vertical angles is that they are always congruent, meaning they have the same measure. This congruency arises because when two lines cross, the angles across from each other share the same vertex and are formed by the same pair of intersecting lines.

In contrast, the other statements about vertical angles do not hold true. For instance, vertical angles do not necessarily sum up to 90 degrees, as they can each be any angle measure that can occur in an intersection, and they are not defined as complementary (which suggests that they must add up to 90 degrees). Additionally, while vertical angles are formed through the intersection of lines, they are not limited to scenarios involving parallel lines; instead, they can occur with any two lines that intersect. Thus, the defining property of vertical angles being congruent makes the first statement the correct one.