where the p i are the momenta of the system and the q i are the corresponding coordinates. The quantum numbers n i are integers and the integral is taken over one period of the motion at constant energy (as described by the Hamiltonian). The integral is an area in phase space, which is a quantity called the action and is quantized in units of the (unreduced) Planck constant. For this reason, the Planck constant was often called the quantum of action.