 One self-loop is labeled with 1, the other with 2, and the edge connecting the two vertices is labeled 0.More generally, the same construction allows any generalized Petersen graph GP("n","k") to be constructed as a derived graph of the same dumbbell graph with labels 1, 0, and "k" in the group ℤ"n".The vertices and edges of any periodic tessellation of the plane may be formed as the derived graph of a finite graph, with voltages in ℤ2.