The most common is to begin numbering the carbons from the carboxylic acid functional group, with that carbon labeled as carbon 1. Following this system, linoleic acid has double bonds at the 9 ...

The carbons and oxygens of the carboxylic acid functional group are red. (B) This is the structure of oleic acid, an 18-carbon unsaturated fat. The carbons of the alkene functional group ...