A grouping within a molecular entity that may be considered a focus of @S05983@. At least one of these must be present in every @E02069@ (though the presence of stereogenic units does not conversely require the corresponding chemical species to be @C01057@). Three basic types are recognized for molecular entities involving atoms having not more than four substituents:
  1. A grouping of atoms consisting of a @C00930@ and distinguishable @L03518@, such that the interchange of any two of the substituents leads to a stereoisomer. An @A00478@ atom (@C01060@) is the traditional example of this stereogenic unit.
  2. A chain of four non-coplanar atoms (or rigid groups) in a @S05900@ @C01258@, such that an imaginary or real (restricted) rotation (with a change of sign of the @T06406@) about the central bond leads to a stereoisomer.
  3. A grouping of atoms consisting of a double bond with substituents which give rise to @C01093@.
PAC, 1996, 68, 2193. (Basic terminology of stereochemistry (IUPAC Recommendations 1996)) on page 2219 [Terms] [Paper]