  The leading twist transverse momentum dependent parton distribution functions
h_{1L}^{\perp} and h_{1T}^\perp, which is sometimes called "pretzelosity," are
studied. For h_{1L}^{\perp} we consider a "Wandzura-Wilczek-type"
approximation, which follows from QCD equations of motion upon the neglect of
pure twist-3 terms and allows to express it in terms of transversity. On the
basis of available data from HERMES we test the practical usefulness of this
approximation and discuss how it can be further tested by future CLAS and
COMPASS data. We review the theoretical properties of retzelosity and 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 the nucleon, is nothing but the
transverse moment of pretzelosity. We discuss preliminary deuteron target data
from COMPASS on the single spin asymmetry (SSA) in semi-inclusive deep
inelastic scattering (SIDIS) related to pretzelosity, and make predictions for
future experiments at JLab, COMPASS and HERMES.
