  Generalized parton distributions describe the distribution of partons in the
transverse plane. For transversely polarized quarks and/or nucleons, these
impact parameter dependent parton distributions are not axially symmetric.
These transverse distortions can be related to spin-orbit correlations as well
as to (intuitively) to transverse single-spin asymmetries, allowing novel
insights into quark orbital angular momentum from measurements of the Sivers
and Boer-Mulders functions.
