  The leading twist transverse momentum dependent parton distribution function
h_1T^perp, which is sometimes called ``pretzelosity,'' is studied. We review
the theoretical properties of this function, and present bag model predictions.
We observe an interesting relation valid in a large class of relativistic
models: The difference between helicity and transversity distributions, which
is often said to be a 'measure of relativistic effects' in nucleon, is nothing
but the pretzelosity distribution. Pretzelosity is chirally odd and can be
accessed in combination with the Collins effect in semi-inclusive deep
inelastic scattering, where it gives rise to an azimuthal single spin asymmetry
proportional to sin(3phi-phi_S). We discuss the preliminary deuteron target
data from COMPASS, on that observable and make predictions for future
experiments on various targets at JLab, COMPASS and HERMES.
