The movie demonstrates several examples of polyhedrons with trihedral or tetrahedral angles.
If several rays have a common beginning and any three ones aren't in the same plane, they form a polyhedral angle. Planar angles at the vertex of the polyhedral angle are called faces, and the rays themselves are called edges, the beginning is called vertex. A polyhedral angle is called trihedral, quadrilateral, etc., depending upon its faces number. If all faces of a polyhedral angle are situated entirely on one side of any of its faces planes, the angle is called convex. We shall consider only convex polyhedral angles.
A number of faces for a demonstrated polyhedron is displayed in a top right cone of the screen.