A set of three mutually perpendicular reflective surfaces, placed to form the internal corner of a cube, work as a retroreflector. The three corresponding normal vectors of the corner's sides form a basis in which to represent the direction of an arbitrary incoming ray, . When the ray reflects from the first side, say x, the ray's "x"-component, "a", is reversed to −"a", while the "y"- and "z"-components are unchanged. Therefore, as the ray reflects first from side x then side y and finally from side z the ray direction goes from to to to and it leaves the corner with all three components of its direction exactly reversed.