 However, when a planar graph covers a non-planar one, the ply must be an even number.The graph of a "k"-gonal prism has 2"k" vertices, and is planar with two "k"-gon faces and "k" quadrilateral faces.If "k" = "ab", with "a" ≥ 2 and "b" ≥ 3, then it has an "a"-ply covering map to a "b"-fonal prism, in which two vertices of the "k"-prism are mapped to the same vertex of the "b"-prism if they both belong to the same "k"-gonal face and the distance from one to the other is a multiple of "b".