  We use the effective Lagrangian formalism to describe quark mixing. The new
W, Z and H couplings generalizing the CKM matrix and the GIM mechanism fulfil
relations and inequalities which allow to discriminate among different SM
extensions. As a by-product we give a useful parametrization of the generalized
CKM matrix. We also show that the largest possible departures from the SM
predictions result from heavy exotic fermions, which can induce, for example,
top FCNC large enough to be observable at future colliders.
