An 180 degree angle formed by two opposite rays is called a straight angle. A ray is a straight line that moves indefinitely in one direction away from a given point. The rays move in opposite directions away from the vortex or meeting point where the angle is formed.
The straight angle is the only type of angle formed by two opposite rays. All other angles formed by two rays are less than 180 degrees. Acute angles are between zero and 90 degrees. A right angle, with one ray perpendicular to another, is 90 degrees. Obtuse angles are between 90 and 180 degrees.