It has been known since the days of Pasteur (famous, notably, for the separation of two forms of tartaric acid) that molecules containing an asymmetric carbon atom, that is, a carbon atom bearing four different groups, can exist in two forms that are to each other like one hand to the other (cheir means hand in Greek) or like an object to its image in a mirror. When aqueous solutions of such molecules are traversed by a beam of polarized light, the polarization plane of the light is rotated by a certain angle. The value of this angle, adjusted to the concentration of the solution and to the thickness of the liquid layer traversed, is known as the specific rotatory power, or optical activity, of the substance; it is the same in absolute value, but of opposite sign, for the two forms. By definition, the optical activity is said to be positive when the polarization plane of the light is rotated to the right, and negative in the opposite case. The two forms, known as enantiomers (enantios means opposite in Greek), are designated d, for dextrorotatory (dexter means right in Latin), and l, for levorotatory (laevus means left in Latin).

Following a proposal made at the beginning of the last century by the German chemist Emil Fischer, the nomenclature based on optical activity was replaced by one based on structure. Stereoisomers (stereos means solid in Greek) replaced optical isomers.