A system used to describe crystal structure. It consists of a capital letter followed by a number. The capital letter indicates the atomic nature of the material. A stands for monoatomic, B for diatomic with equal numbers of atoms of each type, C for diatomic with a ratio of 2:1 for atoms of each type, D is as C but with a ratio of 3:1, E…K are more complicated types of compounds, L are alloys, O are organic compounds and S are silicates. The number gives the order of the discovery of that particular structure. The term is German for ‘structure report’.