POSSIBLE LINKS BETWEEN BL LACERTAE OBJECTS AND QUASARS FROM VERY LONG-BASE-LINE INTERFEROMETRY RADIO DATA

Systematic differences in the very long baseline interferometry (VLBI) radio polarization structure and average VLBI component speeds of BL Lacertae objects and quasars support the view that the observational d istinction between these classes, based in large part on the strength of their optical line emission, is meaningful; in other words, this di stinction reflects significant differences in the physical conditions in these sources. Possible models providing a link between the optical and VLBI properties of BL Lacertae objects and quasars are discussed. Most VLBI polarization observations to date have been global observat ions made at 6 cm; recent results suggest that the VLBI polarization s tructure of some sources may change dramatically on scales smaller tha n those probed by these 6-cm observations.