The polarization electric vectors in the VLBI jets of BL Lacertae objects are typically aligned with the jet structure. If the jet radio emission is optically thin synchrotron emission, this implies that the magnetic field is perpendicular to the jet, usually interpreted as a signature of shocks. The distribution of polarization position angles in the VLBI core components appears to be bimodal, with the polarization angles either aligned with or perpendicular to the jet direction. In order to study the origin of this characteristic polarization structure, we are obtaining VLBI polarization images for all 34 sources in the Kuhr and Schmidt sample of BL Lacertae Objects. New images from this project will be presented and discussed.
(submitted as poster contribution)