The internal structures found in a set of relativistic axisymmetric je
t simulations are analyzed. The structures are induced by a conical pr
essure wave associated with the jet inlet on the computational grid an
d by cocoon vortices around the propagating jet. The observed structur
es and differences between structure in the different simulations are
found to be fully understandable in terms of the structure and growth
or damping of the normal axisymmetric Fourier modes of a cylindrical j
et. Differences in detailed internal structure are largely the result
of differences in the wavelength of perturbations applied by the conic
al pressure wave at the inlet and by cocoon turbulence. In some cases
the perturbations can strongly couple to a normal mode of the jet. In
other cases strong damping occurs when the perturbations cannot easily
couple to a normal mode of the jet. In particular, the relative stabi
lity and lack of strong internal structure in the highly relativistic
numerical simulations performed by Duncan & Hughes is a result of weak
coupling to a normal mode of the jet.