The R/S system is an important nomenclature system for denoting enantiomers. This approach labels each chiral center R or S according to a system by which its substituents are each assigned a priority, according to the Cahn–Ingold–Prelog priority rules (CIP), based on atomic number. When the center is oriented so that the lowest-priority substituent of the four is pointed away from the viewer, the viewer will then see two possibilities: if the priority of the remaining three substituents decreases in clockwise direction, it is labeled R (for  right); if it decreases in counterclockwise direction, it is S (for  left).
(R) or (S) is written in italics and parentheses. If there are multiple chiral carbons, e.g. (1R,4S), a number specifies the location of the carbon preceding each configuration.
The R/S system also has no fixed relation to the  system. For example, the side-chain one of serine contains a hydroxyl group, −OH. If a thiol group, −SH, were swapped in for it, the  labeling would, by its definition, not be affected by the substitution. But this substitution would invert the molecules R/S labeling, because the CIP priority of CHOH is lower than that for COH but the CIP priority of CHSH is higher than that for COH.  For this reason, the  system remains in common use in certain areas of biochemistry, such as amino acid and carbohydrate chemistry, because it is convenient to have the same chiral label for the commonly occurring structures of a given type of structure in higher organisms. In the  system, nearly all naturally occurring amino acids are all , while naturally occurring carbohydrates are nearly all . All proteinogenic amino acids are S, except for cysteine, which is R'.