The basis for Sommerfeld's model is the Wilson-Sommerfeld quantization rule. The state of any
system which carries oscillatory motion can be described by a set of appropriate canonical coordinates
qi which are periodic functions of time. There is a canonical momentum pi associated with each
canonical coordinate qi
. Canonical coordinates and momenta are a convenient way to generalize the
formalism of expressing equations of motion in classical mechanics. For example, regular position and
momentum are good canonical variables describing a free particle, while angle and angular momentum
are good canonical coordinates describing a system constrained to only rotational motion. The WilsonSommerfeld
quantization condition is: