  Many jets exhibit twisted helical structures. Where superluminal motions are
detected, jet orientation and pattern/flow speed are considerably constrained.
In this case modeling efforts can place strong limits on conditions in the jet
and in the external environment. This can be done by modeling the spatial
development of helical structures which are sensitively dependent on these
conditions. Along an expanding jet this sensitivity manifests itself in
predictable changes in pattern speed and observed wavelength. In general,
twists of low frequency relative to the local resonant frequency are advected
along the expanding jet into a region in which the twist frequency is high
relative to the local resonant frequency. The wave speed can be very different
in these two frequency regimes. Potential effects include helical twists with a
nearly constant apparent wavelength, an apparent wavelength scaling
approximately with the jet radius for up to two orders of magnitude of jet
expansion, or multiple twist wavelengths with vastly different intrinsic scale
and vastly different wave speeds that give rise to similar observed twist
wavelengths but with very different observed motion. In this paper I illustrate
the basic intrinsic and observed behavior of these structures and show how to
place constraints on jet conditions in superluminal jets using the apparent
structures and motions in the inner 3C 120 jet.
