  Defining, in the framework of quantum field theory, their mass eigenstates
through their matricial propagator, we show why the mixing matrices of
non-degenerate coupled systems should not be parametrized as unitary. This is
how, for leptonic binary systems, two-angles solutions with discrete values
pi/4 [pi/2] and pi/2 [pi] (in addition to the trivial case 0 [pi]) arise when
weak leptonic currents of mass eigenstates approximately satisfy the two
properties of universality and vanishing of their non-diagonal neutral
components. Charged weak currents are also discussed, which leads to a few
remarks concerning oscillations. We argue that quarks, which cannot be defined
on shell because of the confinement property, are instead more naturally
endowed with unitary Cabibbo-like mixing matrices, involving a single
unconstrained mixing angle. The similarity between neutrinos and neutral kaons
is outlined, together with the role of the symmetry by exchange of families.
