A perpendicular angle is a quarter of a full rotation. In radians, a perpendicular angle is equal to (tau) radians. In degrees, a perpendicular angle is equal to .

An acute angle is an angle that measures less than 90 degrees.

An obtuse angle is an angle that measures more than 90 degrees but less than 180 degrees. It is larger than a right angle and smaller than a straight angle.

A reflex angle is an angle whose measure is greater than 180 degrees but less than 360 degrees.