When one substituent and one hydrogen atom are attached at each of more than two positions of a monocycle, the steric relations of the substituents may be expressed by first identifying a reference substituent (labeled r) followed by a hyphen and the substituent locator number and name. The relative configuration of other substituents are then reported as cis (c) or trans (t) to the reference substituent.
When two different substituents are attached at the same position of a monocycle, then the lowest-numbered substituent named as a suffix is selected as reference group. If none of the substituents is named as a suffix, then that substituent of the pair of substituents having the lowest number, and which is preferred by the sequence rule, is chosen as the reference group. The relationship of the sequence-rule-preferred substituent at geminally substituted positions, relative to the reference group, is cited as c- or t-, as appropriate.
An alternative system which specifies the absolute configuration of substituted carbon atoms may also be used. This system, known as the Cahn-Ingold-Prelog rules, uses and elaborates the priority rules described earlier for stereoisomeric alkenes.'); generator.document.write('