There are four sets of equivalent carbon atoms (see equivalent ligands) in the benzene ring of a monosubstituted benzene.

A = any substituent

C1
C2 ≡ C6
C3 ≡ C5
C4

They are labeled for identification purposes as follows.

A disubstituted benzene could exist as three constitutional isomers (1-3), which differ from one another in the relative positions of the substituents on the benzene ring and can be identified using the descriptors otho, meta, and para.

see also ortho, para directing group, meta directing group