In geometry, a pair of rays (see line ) sharing a common endpoint (the vertex).
An angle may be thought of as the rotation of a single ray from an initial to a terminal position. Clockwise rotation is considered negative and counterclockwise rotation positive. Either may be measured in degrees (one full rotation = 360k) or radians (one full rotation = 2π rad). A 90k angle is called a right angle. Any angle less than 90k is an acute angle. Any angle more than 90k but less than 180k is an obtuse angle.