Study these two figures. Only the pair on the right is considered to be adjacent, angles c and d . Adjacent angles must share a common side and a common vertex, and they must not overlap each other. Vertical angles are pairs of angles formed by two intersecting lines. In this diagram, angles a and c are vertical angles, and angles b and d are vertical angles.
Vertical angles are congruent.
These two lines are parallel, and are cut by a transversal, which is just a name given to a line that intersects two or more lines at different points. Eight angles appear, in four corresponding pairs that have the same measure, so therefore are congruent. The angles that lie in the interior area, or the area between the two lines that are cut by the transversal, are called interior angles. Angles c, d, e and f are interior angles. We call angles on opposite sides of the transversal alternate angles.