Suppose "v", "e", and "f" are the number of vertices, edges, and regions (faces). Since each region is triangular and each edge is shared by two regions, we have that 2"e" = 3"f". This together with Euler's formula, "v" − "e" + "f" = 2, can be used to show that 6"v" − 2"e" = 12. Now, the "degree" of a vertex is the number of edges abutting it. If "v""n" is the number of vertices of degree "n" and "D" is the maximum degree of any vertex,