  Relatively few Very Long Baseline Interferometry (VLBI) polarization
observations have been carried out at 18 cm. The importance of such
observations lies in their ability to reveal information about the jet magnetic
({\bf B}) field structure and the environment of the jet on scales intermediate
between those probed by higher-frequency VLBI and connected-element
interferometers such as the Very Large Array. We have obtained polarization
observations of 34 BL Lac objects with the Very Long Baseline Array (VLBA), at
4 separate wavelengths in the 18-20 cm band. The 18-cm jets typically extend to
tens of parsecs. In some cases, the decaparsec jet is a continuation of the jet
on smaller scales, while in others, we see appreciable bending. We have
constructed Faraday rotation-measure maps and used them to study the jet {\bf
B} field structures and distribution of thermal plasma around the jets. The
Faraday rotation is typically large at these wavelengths, and knowledge of the
rotation-measure distribution is essential to derive the {\bf B} field
structures of the jets. The high sensitivity of these observations to Faraday
rotation makes them an effective tool for studies of possible interactions
between the jets and the media through which they propagate.
