In order to distinguish between the two possibilities, further information must be included in the name of the compound, that is stereochemical terminology is needed. They are: S8 and Sn. Structural isomers may be divided into five sub-types: skeletal, positional, functional, and tautomeric and meta. On a long straight-chained molecule that has a functional group; it is possible for this group to be placed at a number of different positions on the carbon chain. This is referred to as positional isomerism. The two most frequent examples of this type are the enol/keto and the aci/nitro tautomeric systems. The idea of enantiometric molecules may be extended from an atomic centre to a face of a trigonal molecule, by an extension of the Cahn-Ingold-Prelog system. Compounds in which rotation is restricted can exhibit geometric isomerism. A reaction in which only one set of stereoisomers is formed exclusively or predominantly is termed a stereoselective synthesis.