Off-diagonal parton distributions occur in several hard exclusive reactions.
They extend the study of hadron structure beyond what can be learned from
ordinary distributions and have a particularly rich spin structure. The hard
scattering subprocesses in electroproduction of mesons and of real photons
satisfy helicity selection rules, which provide powerful tools to test
leading-twist dominance at a given value of the hard scale.Comment: 7 pages, 3 figures, LaTeX2e. Talk given at the Fourth Workshop on
Quantum Chromodynamics, Paris, France, 1-6 June 199
