Cahn-Ingold-Prelog priority rule
The Cahn–Ingold–Prelog priority rules, CIP system or CIP conventions (after Robert Sidney Cahn
Robert Sidney Cahn
Robert Sidney Cahn was a British chemist, best known for his contributions to chemical nomenclature and stereochemistry, particularly by the Cahn–Ingold–Prelog priority rules, which he proposed in 1956 with Christopher Kelk Ingold and Vladimir Prelog.Cahn was born in Hampstead, London...

, Christopher Kelk Ingold
Christopher Kelk Ingold
Sir Christopher Kelk Ingold FRS was a British chemist based in Leeds and London. His groundbreaking work in the 1920s and 1930s on reaction mechanisms and the electronic structure of organic compounds was responsible for the introduction into mainstream chemistry of concepts such as nucleophile,...

 and Vladimir Prelog
Vladimir Prelog
Vladimir Prelog FRS was a Croatian chemist and Nobel Prize winner in chemistry. Prelog lived and worked in Prague, Zagreb and Zürich during his lifetime.-Biography:...

) are a set of rules used in organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...

 to name the stereoisomers of a molecule. A molecule may contain any number of stereocenter
A stereocenter or stereogenic center is an atom, bearing groups such that an interchanging of any two groups leads to a stereoisomer.A chirality center is a stereocenter consisting of an atom holding a set of ligands in a spatial arrangement which is not superposable on its mirror image...

s and any number of double bonds, and each gives rise to two possible configurations. The purpose of the CIP system is to assign an R or S descriptor to each stereocenter and an E or Z descriptor to each double bond so that the configuration of the entire molecule can be specified uniquely by including the descriptors in its systematic name.