The numbering is done according to the order of priority of the heteroatoms:
O S -N< -N=
Examples:
1,3-Oxazepine
1,2,4-Triazole
As with the functions of aliphatic compounds , the priority function is written in suffix, the others as well as the alkyl groups and halogens, in prefix. Are considered absolutely priority: -CHO carbaldehyde -CN carbonitrile -COOR alkyl carboxylate -COOH carboxylic acid and the like.
Examples:
Pyridine-2-carbaldehyde
Thiophen-3-ol
4-Fluoro-5-methyl-1,3-thiazole-2-carbamide
If the ring is attached to an alkyl chain of several C atoms and carrying a priority function, then it is considered as a substituent:
Example:
2-(2-Isopropyl-5-methylpyridin-4-yl)ethanol