1. In a triangle, the angle between two sides is called the diagonal of this side to the third side. 2. Two nonadjacent corners in a quadrilateral.

An angle can be regarded as a graph formed by a ray rotating from one position to another around its endpoint on a plane. The endpoint of the ray is called the vertex of the angle, and the starting position and ending position of the ray rotation are called the starting edge and ending edge of the angle. A ray rotates counterclockwise around its endpoint O from the original position OA to another position OB, forming an angle α. The ray OA at the beginning of rotation is called the starting edge of angle, and the ray OA at the end of rotation is called the ending edge of angle.