k • p perturbation theory uses the lowest order dispersion relation consistent with the symmetry of the structure to model the electronic bands at or near a single k-point, such as the L-point in IV-VI semiconductors. The k • p model for the L-point with 3m-symmetry is given in the Methods section, where the parameters are categorized into those related to interactions parallel to the L-plane (i.e. in-plane interactions, denoted by ||) and interactions perpendicular to the L-plane (i.e. out-of-plane interactions, denoted by ⊥). The degree to which the bands are inverted is parametrized by M 0 , which we refer to as the "band inversion parameter" since M 0 > 0 yields normal bands (Figure ), M 0 = 0 yields Dirac cones (Figure ), and M 0 < 0 gives inverted bands (Figures and). While both in-plane and out-of-plane interactions are necessary to describe the carrier pockets in e.g. rocksalt SnSe, we begin our discussion with in-plane interactions to understand the role of the band inversion parameter M 0 for valley degeneracy.