 In a one-dimensional lattice the number of reciprocal lattice vectors formula_2 that determine the bands in an energy interval is limited to two when the energy rises.In two and three dimensional lattices the number of reciprocal lattice vectors that determine the free electron bands formula_3 increases more rapidly when the length of the wave vector increases and the energy rises.This is because the number of reciprocal lattice vectors formula_2 that lie in an interval formula_7 increases.