  The fact that not all measurements can be carried out simultaneously is a
peculiar feature of quantum mechanics and responsible for many key phenomena in
the theory, such as complementarity or uncertainty relations. For the special
case of projective measurements quantum behavior can be characterized by the
commutator but for generalized measurements it is not easy to decide whether
two measurements can still be understood in classical terms or whether they
show already quantum features. We prove that generalized measurements which do
not fulfill the notion of joint measurability are nonclassical, as they can be
used for the task of quantum steering. This shows that the notion of joint
measurability is, among several definitions, the proper one to characterize
quantum behavior. Moreover, the equivalence allows to derive novel steering
inequalities from known results on joint measurability and new criteria for
joint measurability from known results on the steerability of states.
