  We demonstrate that there exist two kinds of gauge-invariant decompositions
of the nucleon spin, which are physically inequivalent. The quark and gluon
orbital angular momenta (OAMs) appearing in one decomposition are basically the
canonical OAMs, while the quark and gluon OAMs appearing in another
decomposition are called the dynamical OAMs. It is shown that the dynamical
OAMs of quarks and gluons in the nucleon can be related to definite high-energy
deep-inelastic-scattering observables. On the other hand, we conjecture that
the canonical OAMs of quarks and gluon in the nucleon are model-dependent
quantities, which is meaningful only within a specific theoretical model of the
nucleon.
