  An asymmetric relativistic model for FRII radio sources is described which
takes account of both relativistic effects and intrinsic/environmental
asymmetries to explain the observed structural asymmetry of their radio lobes.
A key feature of the model is jet-sidedness, which can now be determined for
about 80% of the FRII sources in the 3CRR complete sample. It is shown that a
simple asymmetric relativistic model can account for a wide range of
observational data, and that the relativistic and intrinsic asymmetry effects
are of comparable importance. The mean translational speed of the lobes is
0.12c with a standard deviation 0.04c. The results are in agreement with an
orientation-based unified scheme in which the critical angle separating the
radio galaxies from the radio quasars is about 50 degrees.
