= Polyunsaturated fatty acid
A <polyunsaturated fatty acid> is a <fatty acid> with at least two carbon–carbon <double bonds> in its hydrocarbon chain. The carbon–oxygen <double bond> in the <carboxyl group> does not contribute to this count. Many biological examples have cis double bonds, whose geometry changes how chains pack.<Biology 2e by Mary Ann Clark and others>[§3.3, saturated and unsaturated fatty acids]{ref}
For example, <arachidonic acid> has twenty carbons and four carbon–carbon <double bonds>, while <docosahexaenoic acid> has twenty-two carbons and six. Chain length, bond positions and cis/trans geometry convey different information. A count such as “four double bonds” alone cannot uniquely identify the molecule or all of its physical properties.<PubChem Compound database by National Center for Biotechnology Information>[Arachidonic acid, CID 444899; docosahexaenoic acid, CID 445580, structural records]{ref}
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