In IUPAC nomenclature, benzene is designated as a parent name. Other compounds that contain the benzene molecule may be considered as substituted benzenes. In the case of monosubstitution (the replacement of a single hydrogen), the prefix of the substituent is added to the name benzene.
In other cases, the substituent, along with the benzene ring, forms a new parent system.
When a benzene molecule is disubstituted (two hydrogens are replaced), two nomenclature methods exist. Either a number system or name system indicates the relative position of one substituent to the other. In the number system, one substituent is given the number one position and the second substituent is assigned the lower possible second number. The number position is given to the atom or group that has the higher priority as determined by the Cahn‐Ingold‐Prelog nomenclature system rules.