In matrix mechanics, observables such as position and momentum are represented by self-adjoint operators. When considering pairs of observables, an important quantity is the commutator. For a pair of operators Â and B ^ ^ , one defines their commutator as [ A ^ ^ , B ^ ^ ] = A ^ ^ B ^ ^ − − B ^ ^ A ^ ^ . In the case of position and momentum, the commutator is the canonical commutation relation [ x ^ ^ , p ^ ^ ] = i ℏ ℏ .