  We report on the first extraction of interference fragmentation functions
from the semi-inclusive production of two hadron pairs in back-to-back jets in
e+e- annihilation. A nonzero asymmetry in the correlation of azimuthal
orientations of opposite \pi+\pi- pairs is related to the transverse
polarization of fragmenting quarks through a significant polarized dihadron
fragmentation function. Extraction of the latter requires the knowledge of its
unpolarized counterpart, the probability density for a quark to fragment in a
\pi+\pi- pair. Since data for the unpolarized cross section are missing, we
extract the unpolarized dihadron fragmentation function from a Monte Carlo
simulation of the cross section.
