Twist-3 in Proton Nucleon Single Spin Asymmetries

These proceedings contain the 'physical' derivation of the Wandzura-Wilczek relations for the spin-dependent structure function g2 (amounting to keeping the intrinsic transverse momentum of quarks, and neglecting the physical transverse components of the gluon fields), recently generalized for the case of non-forward processes (hep-ph/0102209). The simple numerical model for the twist-3 contribution to g2 was also suggested.