Electronic dispersion of graphene. The conduction band and the valence band touch each other at six discrete points. These points are called points. The six points can be divided into two in-equivalent sets of three points each. The points within each set are all equivalent because they can reach each other by reciprocal lattice vectors. The two in-equivalent points are called and and form the valley isospin degree of freedom in graphene. The name valley isospin stems from the similarity of the vicinity of these points with a valley. The zoom shows that the dispersion relation close to the points looks like the energy spectrum of massless Dirac particles.