Single sheets of hexagonal boron nitride on transition metals provide a model system for single layer dielectrics. The progress in the understanding of h-BN layers on transition metals of the last 10 years is shortly reviewed. Particular emphasis lies on the boron nitride nanomesh on Rh(1 1 1), which is a corrugated single sheet of h-BN, where the corrugation imposes strong lateral electric fields. Fermi surface maps of h-BN/Rh(1 1 1) and Rh(1 1 1) are compared. A h-BN layer on Rh(1 1 1) introduces no new bands at the Fermi energy, which is expected for an insulator. The lateral electric fields of h-BN nanomesh violate the conservation law for parallel momentum in photoemission and smear out the momentum distribution curves on the Fermi surface.