Some functional groups such as alcohols, ketones, amines, alkenes, and alkynes may be bonded to any of several carbon atoms in a carbon chain. These compounds are positional isomers. In order to be able to precisely define the position of these functional groups and also the carbon branches, a number is assigned to each carbon. The numbers should be assigned so that the carbon with the functional group or branch has the LOWEST number. The numbers may go from either left to right or vica versa.

Example top left - Pentenes: For alkenes or alkynes, only the first carbon of the double or triple bond need have a number. Since all on the left are pentenes, it is necessary to know the position of the double bond. The position number is put in front of the name. Notice that the last example is actually the same as the middle example if numbered correctly with the functional group getting the lowest number.

Example lower left - Branched Alkane: All of the carbons in the longest chain are given a number. These position numbers will be used in the next step to give the location of the branches.

Example bottom - Alcohol: The carbons are numbered starting with the carbon with the alcohol functional group.