[en] The authors require the following three conditions to hold on two systems being described as a joint system: (1) the structure of the two systems is preserved; (2) a measurement on one of the systems does not disturb the other one; (3) maximal information obtained on both systems separately gives maximal information on the joint system. With these conditions it is shown, within the framework of the propositional system formalism, that if the systems are classical the joint system is described by the cartesian product of the corresponding phase spaces, and if the systems are quantal the joint system is described by the tensor product of the corresponding Hillbert spaces. (Auth.)