  We discuss the different types of decomposition of the proton spin. We stress
that, despite their lack of uniqueness, the Chen et al. and Wakamatsu
decompositions are perfectly measurable. We argue that a large part of the
recent controversies boils down to the fact that there actually exist two types
of gauge transformations in the Chen et al. approach, where physical and gauge
degrees of freedom of the gauge potential are explicitly separated. By
carefully distinguishing these two types of gauge transformations, one can
easily understand how the concepts of gauge invariance, Stueckelberg symmetry,
path dependence and measurability are linked to each other.
