where M is the chemical symbol of the substrate, A is the chemical symbol of the overlayer, ( h k ℓ ℓ ) are the Miller indices of the surface plane, R and α α correspond to the rotational difference between the substrate and overlayer vectors, and the vector magnitudes shown are those of the substrate ( s subscripts) and of the overlayer ( 0 subscripts). This notation can only describe commensurate overlayers however, while matrix notation can describe both.