  Spin sum rules depend on the choice of a pivot, i.e. the point about which
the angular momentun is defined, usually identified with the center of the
nucleon. The latter is however not unique in a relativistic theory and has led
to apparently contradictory results in the literature. Using the recently
developed phase-space approach, we compute for the first time the contribution
associated with the motion of the center of the nucleon, and we derive a
general spin sum rule which reduces to established results after appropriate
choices for the pivot and the spin component.
