 In formulating a quantum theory of some system, one usually begins with the Hamiltonian or Lagrangian formulation of the classical mechanical theory of that system. In the Hamiltonian formulation of classical mechanics, the configuration of a system is represented by a set of coordinates. These could be, for example, the positions of each of a set of point particles, but one can also consider more general cases, such as angular coordinates that specify the orientation of a rigid body. For every coordinate there is an associated conjugate momentum. If the coordinate indicates the position of some object, the momentum conjugate to that coordinate may be what we usually call “momentum,” that is, the velocity of the body multiplied by its mass. If the coordinate is an angle, the momentum conjugate to it is an angular momentum.