The monomers that make up a lipid are glycerol and fatty acids. The most common lipid structure is the triglyceride, which is comprised of a glycerol backbone bonded to three fatty acid tails.
Fatty acid tails are chains of carbon and hydrogen atoms bonded to a carboxyl group at one end, and a methyl group at the other. The length of a fatty acid tail can be as short as two carbon atoms or as long as more than 20 carbon atoms. The carbon atoms are linked by a single or double bond. The presence of a double bond classifies the fatty acid tail as unsaturated.