  We study duality transformation and duality symmetry in the the
electromagnetic-like charged p-form theories. It is shown that the dichotomic
characterization of duality groups as $Z_2$ or SO(2) remains as the only
possibilities but are now present in all dimensions even and odd. This is a
property defined in the symplectic sector of the theory both for massive and
massless tensors. It is shown that the duality groups depend, in general, both
on the ranks of the fields and on the dimension of the spacetime. We search for
the physical origin of this two-fold property and show that it is traceable to
the dimensional and rank dependence of the parity of certain operator (a
generalized-curl) that naturally decomposes the symplectic sector of the
action. These operators are only slightly different in the massive and in the
massless cases but their physical origin are quite distinct.
