Abstract
Results are presented from our study of ten highly active variable extra-galactic sources observed at multiple frequencies and epochs with the VLBA with the goal of testing whether the observed linear polarization structures are consistent with oblique shock models. We compare the VLBA images of sample member BL Lacertae with the results of simple models which account for relativistic aberration effects, and also with approximately weekly observations obtained with the University of Michigan 26-meter paraboloid, and show that the observed behavior is consistent with the oblique shock hypothesis.