  We present the results of the first survey and monitoring study of the linear
polarisation properties of compact, flat-spectrum radio sources at
millimetre/submillimetre wavelengths and discuss the implications of the
inferred magnetic field structure for the emission models involving shock waves
in relativistic jets on sub-parsec scales. We find significant polarisation in
most sources but, in general, the magnetic field on sub-parsec scales is less
well ordered than on parsec scales. We observe no difference in polarisation
properties between the BL Lac objects and compact flat-spectrum quasars at
these wavelengths. Although we find the behaviour of some sources, particularly
the most highly polarised, to be very consistent with the predictions of
transverse shock-in-jet models, the detailed behaviour of most objects is not.
Conical shock structures can more readily explain the observed diverse
behaviour of the sample, although some degree of bending of the jet may still
be necessary in some cases.
