  Three notions of complementarity - operational, probabilistic, and value
complementarity - are reanalysed with respect to the question of joint
measurements and compared with reference to some examples of canonically
conjugate observables. It is shown that the joint measurability of noncommuting
observables is a consequence of the quantum formalism if unsharp observables
are taken into account; a fact not in conflict with the idea of
complementarity, which, in its strongest version, was originally formulated
only for sharp observables. As an illustration of the general theory, the
wave-particle duality of photons is analysed in terms of complementary path and
interference observables and their unsharp joint measurability.
