  The Mach-Zehnder interferometric setup quantitatively characterizing the
wave-particle duality implements in fact a joint measurement of two unsharp
observables. We present a necessary and sufficient condition for such a pair of
unsharp observables to be jointly measurable. The condition is shown to be
equivalent to a duality inequality, which for the optimal strategy of
extracting the which-path information is more stringent than the
Jaeger-Shimony-Vaidman-Englert inequality.
