Quantum mechanics predicts the joint probability distribution of the outcomes
of simultaneous measurements of commuting observables, but, in the state of the
art, has lacked the operational definition of simultaneous measurements. The
question is answered as to when the consecutive applications of measuring
apparatuses give a simultaneous measurement of their observables. For this
purpose, all the possible state reductions caused by measurements of an
observable is also characterized by their operations.Comment: 9 pages, REVTe