What Are Alternate Angles?

Alternate angles are on opposite sides of the vertex

When transversals cross through two parallel lines, they create two identical sets of angles. The two angles that would be opposite the vertex to each other (if they were in the same set) are called alternate angles.

Alternate angles are basically vertical to each other, so if the lines the transversal crosses through are parallel they will always be congruent.

In the image above, the angles that share the same color mark are alternate to each other (for example, EYB and CZF).