Editing Cahn–Ingold–Prelog priority rules (section)

== Describing multiple centers ==
If a compound has more than one chiral stereocenter, each center is denoted by either ''R'' or ''S''. For example, [[ephedrine]] exists in (1''R'',2''S'') and (1''S'',2''R'') stereoisomers, which are distinct mirror-image forms of each other, making them [[enantiomer]]s. This compound also exists as the two [[enantiomer]]s written (1''R'',2''R'') and (1''S'',2''S''), which are named [[pseudoephedrine]] rather than ephedrine. All four of these isomers are named 2-methylamino-1-phenyl-1-propanol in systematic nomenclature. However, ephedrine and pseudoephedrine are [[diastereomers]], or stereoisomers that are not enantiomers because they are not related as mirror-image copies. Pseudoephedrine and ephedrine are given different names because, as diastereomers, they have different chemical properties, even for [[racemic mixture]]s of each.

More generally, for any pair of enantiomers, all of the descriptors are opposite: (''R'',''R'') and (''S'',''S'') are enantiomers, as are (''R'',''S'') and (''S'',''R''). Diastereomers have at least one descriptor in common; for example (''R'',''S'') and (''R'',''R'') are diastereomers, as are (''S'',''R'') and (''S'',''S''). This holds true also for compounds having more than two stereocenters: if two stereoisomers have at least one descriptor in common, they are diastereomers. If all the descriptors are opposite, they are enantiomers.

A [[meso compound]] is an achiral molecule, despite having two or more [[stereogenic center]]s. A meso compound is superposable on its mirror image, therefore it reduces the number of stereoisomers predicted by the 2<sup>n</sup> rule. This occurs because the molecule obtains a plane of symmetry that causes the molecule to rotate around the central carbon–carbon bond.<ref name=":1" /> One example is meso-[[tartaric acid]], in which (''R'',''S'') is the same as the (''S'',''R'') form. In meso compounds the ''R'' and ''S'' stereocenters occur in symmetrically positioned pairs.<ref>{{Cite book |author=Bruice, Paula Yurkanis |url=http://worldcat.org/oclc/1046519135 |title=Organic chemistry |date=2007 |publisher=Pearson Prentice Hall |isbn=978-0-13-199631-1 |oclc=1046519135}}</ref>