An face is a topological entity, see fig. 1, which points to four edges, Edge0Name, Edge1Name, Edge2Name and Edge3Name. Figure 1 shows the configuration of a face, with its four associated edges. Note that two adjacent edges must share a common vertex; hence, the face is also associated with four distinct vertices.
The order in which the vertices of each edge are defined is unimportant. Two adjacent edges, however, must share a common vertex. For instance, in fig. 1, edges EdgeAB and EdgeAC share a common vertex, VertexA. Clearly, the face is then associated with four vertices v0 = VertexC, v1 = VertexA, v2 = VertexB and v3 = VertexD. In it not necessary to define the vertices since they are inherited from the definition of the edges. By definition, vertex v0 is the common vertex of edges e3 and e0, v1 that of edges e0 and e1, v2 that of edges e1 and e2, and v3 that of edges e2 and e3. Note that the edges must be defined in a cyclic order while rotating around the face. It then becomes possible to assign a normal to the face: based on the right hand rule, the normal to the face in fig. 1 points out of the plane of the sceen.