In Euclidean geometry, 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 formed by two rays lie in the plane that contains the rays. The intersection of two planes also forms Angles. These are called dihedral angles. Angle is also used to designate the measure of an angle or a rotation. This measure is the ratio of the length of a circular arc to its radius. In a geometric angle, the arc is centered at the vertex and delimited by the sides. In a rotation, the arc is centered at the center of the rotation and delimited by any other point and its image by the rotation.