1The (+) and (-) symbols denote the direction in which polarized light is rotated by the compound and historically constitute the oldest convention used to differentiate between optically active stereoisomers. The dextro- and laevo- prefixes identify the (+) and (-) forms, respectively. The D and L symbols denote chirality as defined by Emil Fischer, i.e. in reference to D- and L-glucose, so that the D series is derived from (+)-glucose and the L series from (-)-glucose. However, not all D compounds are dextro- and not all L need be laevo-. Finally, the modern Cahn-Ingold-Prelog convention identified absolute configuration on carbon as either R or S, and absolute configuration need not correspond to the sense of optical rotation. This essay tracks the evolution of these conventions.