The quark helicity measured in polarized deep inelastic scattering is different from the quark spin in the rest frame of the nucleon. We point out that the quark spin distributions ∆qRF (x) are connected with the quark helicity distributions ∆q(x) and the quark transversity distributions δq(x) by an approximate relation: ∆qRF (x) +∆q(x) = 2δq(x). This relation will be useful in order to measure...
