The relationship between lattice-types may be shown in a particularly instructive manner if W erner 's coordination theory be applied to the description of crystal structures as has already been done by P feiffer . If this be carried out in a systematic fashion and with due consideration to appropriate stoichiometric proportions between central atoms and ligands, a large group of structures is obtained the central atom of which is in the negative state of oxydation. Examples with oxygen as coordination centre confirm the value of this method for systematic purposes.