Figure 1. A classical periodic orbit, \(x(t)\), can be represented as a Fourier sum of “harmonics” which are integer multiples of the fundamental frequency, \(\omega\), representing the periodicity of the motion. According to the selection rule interpretation, the correspondence principle is Bohr’s insight that each allowed transition between stationary states corresponds to one harmonic component of the classical motion. (Based on Fig. 3 of Fedak and Prentis 2002)