. Most evidence supports a stereochemical theory of odor. Amoore et al. (in 1952) proposed that the sense of smell is based largely on the geometry of the odorant molecules. In this theory, there are a small number of primary odors that are detected by complementary receptor sites in the nose. Molecules that fit into a similar primary odor family do not necessarily have similar structural formulas, but they do have roughly the same molecular shape and size. Sometimes, charge is an additional important component. When a molecule of the correct size and shape fits into a complementary receptor site, an impulse is initiated (Figure 1). Complex odors result when a molecule fits into more than one kind of site (i.e., sideways into a wide receptor site and end-on into a narrow receptor site).

X-ray diffraction, infrared spectroscopy, and electron-beam probes have enabled scientists to build models of the seven primary odors.