This chapter extends our investigation into the structures and naming of essential oil compounds. The differences between isomers can be more subtle than this, perhaps relating to the different positioning of a double bond. Geometric isomers are associated with carbon double bonds. Living organisms seem to preferentially produce or utilize only one enantiomer of pairs of enantiomeric molecules. An easy way to detect whether a pure liquid contains chiral carbon atoms is to test its optical activity in polarised light. Most essential oils display some optical activity due to the different combinations of enantiomeric molecules they contain. Alpha-terpinene is an example of the common name of a terpenoid molecule. Its systematic chemical name is 1-methyl-4-isopropyl-1,3-cyclohexadiene. Another term, usually found in relation to molecules which have benzene rings, is ‘para-’, or sometimes just ‘p-’.