Band warping at a k-point also depends on the coupling strength between the band edge states and secondary bands. We define "secondary bands" as states which are neither the valence band edge nor conduction band edge (Figure ). Whether an electronic state is coupled to another depends on the symmetries of the states, similar to how selection rules for optical transitions are determined from the wave function symmetries of the initial and final states. Every state can be labeled by symmetry at a given k-point; if the k-point is invariant under crystal inversion,