  Hadronization corrections to the thrust and related event shape distributions
in the two-jet kinematical region of e+e- annihilation are summarized by
nonperturbative shape functions. The moments of shape functions are given by
universal matrix elements in QCD, which describe the energy flow in QCD final
states. We show how the nonperturbative structure of these matrix elements may
be inferred from resummed perturbation theory and Lorentz invariance. This
analysis suggests the same functional forms for the shape functions as were
found in phenomenological studies, and sheds light on the physical significance
of the parameters that characterize these functions.
