Structural studies conducted in several materials, as well as electron counting arguments; suggest that at least some of the compounds are commensurate or close to commensurability. Commensurability and near commensurability effects have been searched for several materials; clear evidence for these effects, such as a gap in the phason excitation spectrum or nonlinear excitations at energies below the single particle gap. The collective excitations related to the phase remain gapless. The form of the localized excitation can be different in the two subbands, generally leading to both charge and spin excitations. In several materials which have a charge or spin density wave ground state, the period of the density wave is commensurate or nearly commensurate with the underlying lattice. The periodic lattice potential also leads to important, but somewhat different, effects when the density wave is not entirely, but only nearly, commensurate with the underlying lattice.