The chemical names are given in accordance with the rules laid down by the International Union of Pure and Applied Chemistry (IUPAC). In many cases, when equally acceptable alternative names may be construed under these rules, more than one systematic name is given. Such alternative names are given especially when changes in the interpretation of IUPAC rules occurring in recent years have led to substantial modifications of the chemical names used for the substance. The recognition of substances is further facilitated by a registry number established by the Chemical Abstracts Service of the American Chemical Society (CAS No.).