  Using Drell-Yan pair production as an example, we explore the relation
between two well-known mechanisms for single transverse-spin asymmetries in
hard processes: twist-three quark-gluon correlations when the pair's transverse
momentum is large, $q_\perp \gg \Lambda_{\rm QCD}$, and time-reversal-odd and
transverse-momentum-dependent parton distributions when $q_\perp$ is much less
than the pair's mass. We find that although the two mechanisms have their own
domain of validity, they describe the same physics in the kinematic region
where they overlap. This unifies the two mechanisms and imposes an important
constraint on phenomenological studies of single spin asymmetries.
