from Greek pará for adjoining, to the side) describe the three possible positional isomers of two substituents on a benzene ring. These are usually two independent single substituents, but in case of fused ring systems, ortho-fusing is also mentioned unless the substitution pattern is regarded in the name like in [2.2]paracyclophane. In the current systematic nomenclature, o-, m- and p- are often replaced by using locants (1,2-dimethylbenzene instead of o-xylene). o-, m- and p- (written out ortho-, meta- and para-) are written in lowercase letters and italic. exo, endo See: Endo-exo isomerism exo (from Greek = outside) or endo (from Greek endon = inside) denotes the relative configuration of bridged bicyclic compounds. The position of a substituent in the main ring relative to the shortest bridge is decisive for the assignment of exo or endo (according to IUPAC: the bridge with the highest locant digits in the bridged ring system). The substituent to be classified is attributed with the exo descriptor when facing the bridge. It is endo configured when facing away from the bridge. If two different substituents are located on the same C atom, the exo/endo assignment is based on the substituent with higher priority according to the CIP rules. syn, anti If a bridged bicyclic system carries a substituent at the shortest bridge, the exo or endo descriptor can not be used for its assignment. Such isomers are classified by the syn/anti notation. If the substituent to be assigned points towards the ring with the highest number of segments it is syn configured (from Greek syn = together). Otherwise it is attributed with the anti descriptor (Greek anti = against). If both rings possess an equal number of segments the ring with the most significant substituent according to the CIP rules is chosen. The use of syn and anti to indicate the configuration of double bonds is nowadays obsolete, especially in case of aldoximes and aldehydes derived from hydrazones. Here, the compounds were designated as syn configured when the aldehyde H and the O (of the oxime) or the N (of the hydrazone) were cis aligned. These compounds are now described by the (E)/(Z) nomenclature. Aldoximes and hydrazones classified as syn are therefore by now described as (E) configurated. When talking of diastereomers, syn and anti are used to describe groups on the same or opposite sites in zigzag prijection, see Diastereomer#Syn / anti syn and anti