This is a report on an ongoing project designed to study structural variations on the milliarcsecond scale in four BL Lac-type objects. The observations are made at λ6 cm with a global VLBI network consisting of the combined US and European networks. Hybrid maps at three epochs are presented for each of the sources AO 0235+164,0735+178, Mk 421, and 1749+701.
+ Discussion on page 435.