Comment on The number of permitted modes of propagation in N-beam dynamical diffraction of X-rays by S. L. Chang

Attention is directed to a previous treatment of multi-beam diffraction of X-rays by Colella [Acta Cryst. (1974), A30, 413-423]. The rules governing the number of permitted modes are discussed and compared with those given in a recent article by Chang [Acta Cryst. (1979), A35, 543-547].