  Based on gauge-invariant decomposition of covariant angular momentum tensor
of QCD in an arbitrary Lorentz frame, we investigate the relation between the
known decompositions of the nucleon spin into its constituents, thereby
clarifying in what respect they are common and in what respect they are
different critically. We argue that the decomposition of Bashinsky and Jaffe,
that of Chen et al., and that of Jaffe and Manohar are contained in our more
general decomposition, after an appropriate gauge-fixing in a suitable Lorentz
frame, which means that they all {\it gauge-equivalent}. We however point out
that there is another gauge-invariant decomposition of the nucleon spin, which
is closer to the Ji decomposition, while allowing the decomposition of the
gluon total angular momentum into its spin and orbital parts. An advantage of
the latter decomposition is that each of the four terms corresponds to a
definite observable, which can be extracted from high-energy
deep-inelastic-scattering measurements.
