An angle is the figure formed by two rays, called the sides of the angle, sharing a common endpoint, called the vertex of the angle. Angles are usually used in Euclidean plane or in Euclidean space, but are also applied in non-Euclidean geometry. In particular, in spherical geometry the spherical angles are defined using arcs of great circles instead of rays. Also the angle is the measure of the angle or rotation. It is the ratio of arc length to radius. The radian is the standard SI unit with symbol «rad» and «C».