The most basic aryl group is phenyl, which is made up of a benzene ring with one of its hydrogen atom replaced by some substituent, and has the molecular formula . Note that a phenyl group is not the same as a benzyl group, the latter consisting of a phenyl group attached to a methyl group and a molecular formula of .
To name compounds containing phenyl groups, the phenyl group can be taken to be the parent hydrocarbon and be represented by the suffix  Alternatively, the phenyl group could be treated as the substituent, being described within the name as "phenyl". This is usually done when the group attached to the phenyl group consists of six or more carbon atoms.
As an example, consider a hydroxyl group connected to a phenyl group. In this case, if the phenyl group were taken to be the parent hydrocarbon, the compound would be named hydroxybenzene. Alternatively, and more commonly, the hydroxyl group could be taken as the parent group and the phenyl group treated as the substituent, resulting in the more familiar name phenol.