(a) the relation (1.7) that Planck postulated to explain the quantization of energy, E = nhv , and (b) the condition (1.63) that Bohr postulated to account for the quantization of the electron's orbital angular momentum , L = nh. A number of attempts were undertaken to understand or interpret these rules. In 1916 Wilson and Sommerfeld offerd a scheme that included both quantization rules as special cases. In essence, their scheme, which applies only to systems with coordinates that are periodic in time, consists in quantizing the action variable, of classical mechanics.