TMDs are an extension of the concept of parton distribution functions (PDFs) and structure functions that are measured in deep inelastic scattering (DIS). Some TMDs provide the k T dependence of the probabilities that the PDFs represent and that give rise to the DIS structure functions, namely the quark momentum probability distribution f 1 q ( x ) for the unpolarized structure function F 1 ( x ) and the quark spin probability distribution g 1 q ( x ) for the polarized structure functions g 1 ( x ) . Here, x denotes the fraction of hadron longitudinal momentum carried by the parton, and identifies with the Bjorken scaling variable in the infinite energy-momentum limit. The f 1 q ( x ) and g 1 q ( x ) PDFs are summed over all the k T values, and therefore, the k T -dependence of the probabilities is integrated out. TMDs provides the unintegrated probabilities, with their k T -dependence. Other TMDs exist that are not directly connected to f 1 q ( x ) and g 1 q ( x ) . In all, there are 16 dominant (viz leading-twist) independent TMDs, 8 for the quarks and 8 for the gluons.