 Additionally, a disjoint union of two graphs that have covers will also have a cover, formed as the disjoint union of the covering graphs.If the two covers have the same ply as each other, then this will also be the ply of their union.If a graph "H" has an embedding into the projective plane, then it necessarily has a planar cover, given by the preimage of "H" in the orientable double cover of the projective plane, which is a sphere.