Semisymmetric graphs from polytopes

Every finite, self-dual, regular (or chiral) 4-polytope of type { 3 , q , 3 } has a trivalent 3-transitive (or 2-transitive) medial layer graph. Here, by dropping self-duality, we obtain a construction for semisymmetric trivalent graphs (which are edge- but not vertex-transitive). In particular, the Gray graph arises as the medial layer graph of a certain universal locally toroidal regular 4-polytope.