  We present the results of simultaneous VLBA polarisation observations of the
BL Lacertae object 0820+225 at 5, 8, and 15 GHz, together with earlier images
at 5 GHz. This source has an unusually rich total intensity and polarisation
structure compared to other objects with comparable redshifts. The magnetic
field in the inner part of the complex and highly twisted VLBI jet is
transverse, as is typical of BL Lacertae objects, but becomes roughly
longitudinal further from the core, possibly due to shear. Although the
integrated rotation measure of 0820+225 is modest, the rotation-measure
distribution on parsec scales is non-uniform, and clearly shows regions where
the rotation measure is substantially higher than the integrated value.
