  Surface/edge energy is typically modeled as a continuous function of
orientation, $\gamma({\bf n})$. We put forward a simple geometric argument that
suggests this picture is inadequate for crystals with a non-Bravais lattice
structure. In the case of the idealized graphene/hexagonal lattice, our
arguments indicate that the edge energy can be viewed as both discontinuous and
multi-valued for a subset of orientations that are commensurate with the
crystal structure.
