  We present the QCD analysis of D-parameter distribution in near-to-planar
3-jet e+e- annihilation events. We derive the all-order resummed perturbative
prediction and the leading power suppressed non-perturbative corrections both
to the mean value and the distribution. Here non-perturbative corrections are
larger than in 2-jet shape observables, so that higher order non-perturbative
effects could be relevant. Experimental data (not yet available) are needed in
order to cast light on this important point. The technique we develop aims at
improving the accuracy of the theoretical description of multi-jet ensembles,
in particular in hadron-hadron collisions.
