The energies in an energy band in a disordered solid, denoted E_c, at which there is a transition between localized states (see localization), which do not contribute to the electrical conductivity of the system at absolute zero, and extended states, which can contribute to the electrical conductivity. For a given band there are two mobility edges, with the extended states towards the middle of the band and the localized states in the ‘tails’ of the band. In a three-dimensional disordered solid the mobility edges move towards the centre of the band as the disorder increases, until eventually the mobility edges meet and all states become localized. A mobility edge is analogous to a transition temperature in a phase transition.