Angles are formed by two rays that begin at the same point or share the same endpoint. The angle measures the amount of turn between the two arms and is usually measured in degrees or radians. Where the two rays intersect (come together to meet) is called the vertex. The two rays are sometimes referred to as the arms. Angles are usually named using alphabet letters such as (angle symbol) ABC, DEF (Any consecutive letters.) Angles that are exactly 90 degrees are called right angles. Angles less than 90 degrees are called as acute angles. Angles that are exactly 180 degrees is called as straight angle. Angles less than 180 degrees yet greater than 90 degrees are called as obtuse angles.