  We present the main theoretical aspects of the quantum mechanical
string+${}^3P_0$ model of polarized quark fragmentation for pseudoscalar and
vector meson production. The model is an extension of the Lund string
fragmentation model that systematically includes the quark spin degree of
freedom assuming the ${}^3P_0$ mechanism of quark pair production at string
breaking. The recently achieved introduction of vector mesons accounts for the
spin correlations between the latter and the quark of the fragmentation chain.
The string+${}^3P_0$ model reproduces transverse spin effects such as the
Collins effect and the dihadron asymmetry, and longitudinal spin effects such
as the jet handedness.
