  A group theoretical procedure for parametrizing non-forward matrix elements
of non-local QCD operators is introduced. The related (two-variable)
distribution amplitudes are given as sum over power corrections of the double
distributions. For totally symmetric (local) operators the infinite twist
decomposition is given completely and for operators with non-trivial symmetry
type up to twist 3. The power corrections for most of the phenomenological
relevant double distributions and the vector meson wave functions are
determined. They may be obtained, by harmonic extension, from the corresponding
operators or distribution amplitudes on the light-cone.
