Our initial understanding of the short-distance nucleon structure has come from deep inelastic scattering (DIS) experiments. This description is essentially one-dimensional: DIS provides us with the parton momentum distributions in term of the single variable x, which is interpreted in the infinite momentum limit (the Bjorken limit) as the fraction of the nucleon momentum carried by the struck partons. Therefore, from DIS we only learn about the relative longitudinal momentum distribution of the partons, i.e. their longitudinal motions inside the nucleon.