A descriptor is in chemical nomenclature a prefix placed before the systematic substance name, which describes the configuration or the stereochemistry of the molecule. Some listed descriptors are only of historical interest and should not be used in publications anymore as they do not correspond with the modern recommendations of the IUPAC. Stereodescriptors are often used in combination with locants to clearly identify a chemical structure unambiguously. The descriptors, usually placed at the beginning of the systematic name, are not taken into account in the alphabetical sorting. Configuration descriptors cis, trans See: cis–trans isomerism The descriptors cis (lat. on this side of) and trans (lat. over, beyond) are used in various contexts for the description of chemical configurations: In organic structural chemistry, the configuration of a double bond can be described with cis and trans, in case it has a simple substitution pattern with only two residues. The position of two residues relative to one another at different points in a ring system or a larger molecule can also be described with cis and trans if the structure's configuration is rigid and does not allow simple inversion. In inorganic complex chemistry, the descriptors cis and trans are used to characterize the positional isomers in octahedral complexes with A2B4X configuration or square planar complexes with A2B2X configuration. The typographic presentation of cis and trans is italicised and in lower case letters. The cis/trans nomenclature is not unambiguous for more highly substituted double bonds and is nowadays largely replaced by the (E)/(Z) nomenclature. (E), (Z) See: E-Z notation The descriptors (E) (from German entgegen, opposite) and (Z) (from German zusammen, together) are used to provide a distinct description of the substitution pattern for alkenes, cumulenes or other double bond systems such as oximes. For the attribution of (E) or (Z) is based on the relative position of the two substituents of highest priority are on each side of the double bond, while the priority is based on the CIP nomenclature. The (E)/(Z) nomenclature can be applied to any double bond systems (including heteroatoms), but not to substituted ring systems. The descriptors (E) and (Z) are always capitalized, set italic, and surrounded by parentheses that are set as normal just like additional locants or commas. o-, m-, p- See: Arene substitution pattern The abbreviation o- (short for ortho, from Greek orthós for upright, straight), m- (meta, Greek (roughly) for between) and p- (para,