  The proton spin puzzle issue focused the attention on the parton spin and
orbital angular momentum contributions to the proton spin. However, a complete
characterization of the proton spin structure requires also the knowledge of
the parton spin-orbit correlation. We showed that this quantity can be
expressed in terms of moments of measurable parton distributions. Using the
available phenomenological information about the valence quarks, we concluded
that this correlation is negative, meaning that the valence quark spin and
kinetic orbital angular momentum are, in average, opposite. The quark
spin-orbit correlation can also be expressed more intuitively in terms of
relativistic phase-space distributions, which can be seen as the mother
distributions of the standard generalized and transverse-momentum dependent
parton distributions. We present here for the first time some examples of the
general multipole decomposition of these phase-space distributions.
