them. For example, the angle with vertex A formed by the rays AB and AC (that is, the lines from point A to points B and C) is denoted or . Where there is no risk of confusion, the angle may sometimes be referred to by its vertex (in this case "angle A"). Potentially, an angle denoted as, say, , might refer to any of four angles: the clockwise angle from B to C, the anticlockwise angle from B to C, the clockwise angle from C to B, or the anticlockwise angle from C to B, where the direction in which the angle is measured determines its sign (see ). However, in many geometrical situations, it is obvious from context that the positive angle less than or equal to 180 degrees is meant, in which case no ambiguity arises. Otherwise, a convention may be adopted so that always refers to the anticlockwise (positive) angle from B to C, and the anticlockwise (positive) angle from C to B. Types of angles Individual angles There is some common terminology for angles, whose measure is always non-negative (see ): An angle equal to 0° or not turned is called a zero angle. An angle smaller than a right angle (less than 90°) is called an acute angle ("acute" meaning "sharp"). An angle equal to turn (90° or radians) is called a right angle. Two lines that form a right angle are said to be normal, orthogonal, or perpendicular. An angle larger than a right angle and smaller than a straight angle (between 90° and 180°) is called an obtuse angle ("obtuse" meaning "blunt"). An angle equal to turn (180° or radians) is called a straight angle. An angle larger than a straight angle but less than 1 turn (between 180° and 360°) is called a reflex angle. An angle equal to 1 turn (360° or 2 radians) is called a full angle, complete angle, round angle or perigon. An angle that is not a multiple of a right angle is called an oblique angle. The names, intervals, and measuring units are shown in the table below: Equivalence angle pairs Angles that have the same measure (i.e. the same magnitude) are said to be equal or congruent. An angle is defined by its measure and is not dependent upon the lengths of the sides of the angle (e.g. all right angles are equal in