Nomenclature when a  Presence of identical heteroatoms (Multiplicity of heteroatoms)

When two or more heteroatoms of the same type are present in a ring, the prefixes, di-, tri-, tetra, etc. are used and placed before the prefix used for the heteroatom.

Presence of different heteroatoms (Order of preference)

When two or more different heteroatoms are present in the same ring, the prefixes of heteroatoms are combined in the order of their appearance in Table 1.

With two or more identical heteroatoms : When two or more identical heteroatoms are present in a ring, the ring is numbered in such a way that the heteroatoms are assigned the lowest possible set of number locants.

With two or more different heteroatoms : When heterocyclic ring contains two or more different heteroatoms, the numbering starts from the heteroatom with the highest preference as in Table 1 : 0, S, N (oxygen takes precedence over sulfur and sulfur over nitrogen). The remaining heteroatomsare given the lowest number locants.

However, the heterocyclic system in which a carbon atom of the ring is involved in the carbonyl group, the indicated hydrogen is normally cited as an etalic capital H in parentheses after the locant of the additional structural feature.