The theorem can be illustrated with a particle coupled to a measuring apparatus. If the position operator of the particle is q and its momentum operator is p , and if the position and momentum of the apparatus are Q and P respectively, assuming that the total momentum p + P is conserved implies that, in a suitably quantified sense, the particle's position itself cannot be measured. The measurable quantity is its position relative to the measuring apparatus, represented by the operator q − − Q . The Wigner–Araki–Yanase theorem generalizes this to the case of two arbitrary observables A and B for the system and an observable C for the apparatus, satisfying the condition that B + C is conserved.