VLA obseravtions of a complete sample of core-dominated radio source.

Maps with high dynamic range are presented of a well-defined sample of powerful core-dominated radio sources, all of which have 5-GHz core flux densities > 1 Jy. The maps were made at a frequency of 1.64 GHz from combined VLA A and B configuration data. A novel technique for producing high dynamic range maps of variable sources from data taken at different epochs is described. It is found that, on average, BL Lac objects are not more core-dominated than the quasars in the sample. This provides no support for the view that BL Lacs are those quasars seen at such small angles to the line of sight that their relativistically beamed core emission swamps that from other components. The results are consistent with most BL Lac objects being the beamed cores of low-luminosity (FRI) radio galaxies. Nevertheless, a substantial fraction of BL Lac objects, especially those with redshifts > 0.5, have extended radio emission whose luminosity is equivalent to that of high-luminosity (FRII) radio galaxies. The existence of these objects has serious implications for `unified schemes'. Amongst the broad-line objects we find no evidence that those classified as highly polarized or optically violently variable are any more core-dominated than the rest.