  The proton's transverse polarization structure is examined in terms of the
Lorentz-covariant Pauli-Lubanski vector in QCD. We find that there are
contributions from leading, subleading, and next-to-subleading partonic
contributions in the light-front system of coordinates. The subleading and
next-to-subleading contributions are related to the leading one through Lorentz
symmetry. And the leading contribution obeys a simple partonic angular momentum
sum rule that gives a clear physical interpretation to a relation known
previously.
