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The Cahn–Ingold–Prelog priority rules, abbreviated CIP, are the standard method in stereochemistry for assigning absolute configuration to stereogenic centers and for describing geometric isomerism in alkenes. Developed by Robert S. Cahn, Christopher Ingold, and Vladimir Prelog in the mid-20th century, the system provides a consistent way to label chiral centers as R or S and alkenes as E or Z.

The assignment begins by ranking substituents attached to a stereogenic center according to atomic number. The atom directly bonded to the stereocenter with the highest atomic number receives the highest priority. If there is a tie, the comparison proceeds along the substituent chains atom by atom; multiple bonds are treated as if the bonded atoms are duplicated, and isotopes outrank lighter isotopes. The tie is broken when a difference is found along the substituent paths.

To determine R or S, arrange the molecule so that the lowest-priority substituent is directed away from the observer. Trace a path from the highest priority (1) to the second (2) and then to the third (3). If this path moves clockwise, the configuration is R; if it moves counterclockwise, it is S. If the lowest-priority group is not away from the viewer, the resulting designation must be inverted.

For alkenes, the same prioritization is applied to the substituents on each carbon of the double bond. If the high-priority groups are on opposite sides of the double bond, the isomer is E; if they are on the same side, the isomer is Z.

The CIP rules are widely used in chemistry literature and nomenclature, with extensions for rings, fused systems, and specialized cases.