The composition of a mixture of enantiomers may be measured and described as a numerical value. The particular notation depends on the measurement procedure.

In early times optical rotation was the only property amenable to measurement. The outcome of such a measurement was the specific rotation of a compound sample

[] = 100  / (c·d) ,

where is the measured angle of rotation (°), c is the concentration of the sample solution (g/100 ml solution), and d is the length (dm) of the cuvette containing the sample solution. The product c·d is proportional to the mass amount of solute which interacts with the beam of polarized light, it appears in the denominater in order that [] be proportional to the rotatory power of a constant amount. Since the rotatory power is dependent on the wavelength of light used and on the temperature, these parameters have to be given, e.g. in the form []_D²⁰. D here is the wavelength of the sodium D line (589 nm), 20 stands for 20°C. Further the rotatory power depends on solvent and concentration, so that these have also to be given (even though a division by c was already done, the reason for that are concentration-dependent association phenomena).

Example: []_D²⁰ = -13.5 (c=1.3; ethanol) for a particular sample of (R)-2-butanol. The minus sign means laevorotatory, i.e. counterclockwise if the observer's view is opposite to the incoming beam of light.