 According to the selection rule interpretation, Bohr’s correspondence principle is best understood as the statement that each allowed quantum transition between stationary states corresponds to one harmonic component of the classical motion. More precisely, Bohr’s selection rule states that the transition from a stationary state \(n'\) to another stationary state \(n''\) is allowed if and only if there exists a \(\tau\)th harmonic in the classical motion of the electron in the initial stationary state; if there is no \(\tau\)th harmonic in the classical motion, then transitions between stationary states whose separation is \(\tau\) are not allowed quantum mechanically. The essence of Bohr’s correspondence principle is depicted in Figure 1.