  In order to unravel the origin of the nucleon spin, one has to study in
detail the question of orbital angular momentum, and in particular the
reference point about which it is defined. With this in mind, we review the
concept of relativistic center of mass, generalize the discussion to the case
of asymmetric energy-momentum tensors, and establish the link with the
light-front formalism. We find that the $p$-wave in the Dirac plane-wave
solutions arises from a relativistic quantum-mechanical effect which forces the
canonical reference point to depend on the observer. This explains why
longitudinal spin is much simpler to study than transverse spin. It is also the
reason behind the observation of induced shifts and distortions in the parton
distributions defined within the light-front formalism.
