  A model for dihadron fragmentation functions is briefly outlined, that
describes the fragmentation of a quark in two unpolarized hadrons. The
parameters are tuned to the output of the PYTHIA event generator for two-hadron
semi-inclusive production in deep inelastic scattering at HERMES. Then,
predictions are made for the unknown polarized fragmentation function and the
related single-spin asymmetry in the azimuthal distribution of $\pi^+ \pi^-$
pairs in semi-inclusive deep inelastic scattering on transversely polarized
targets at HERMES and COMPASS. This asymmetry can be used to extract the quark
transversity distribution.
