  Relativistic phase space distributions are very interesting objects as they
allow one to gather the information extracted from various types of experiments
into a single coherent picture. Focusing on the four-dimensional transverse
phase space, we identified all the possible angular correlations providing at
the same time a clear physical interpretation of all the leading-twist
generalized and transverse-momentum dependent parton distributions. We also
developed a convenient representation of this four-dimensional space.
