  We study power corrections to the differential thrust, heavy mass and related
event shape distributions in $e^+e^-$-annihilation, whose values, $e$, are
proportional to jet masses in the two-jet limit, $e\to 0$. The factorization
properties of these differential distributions imply that they may be written
as convolutions of nonperturbative "shape" functions, describing the emission
of soft quanta by the jets, and resummed perturbative cross sections. The
infrared shape functions are different for different event shapes, and depend
on a factorization scale, but are independent of the center-of-mass energy $Q$.
They organize all power corrections of the form $1/(eQ)^n$, for arbitrary $n$,
and carry information on a class of universal matrix elements of the
energy-momentum tensor in QCD, directly related to the energy-energy
correlations.
