  Relativistic jets in Galactic superluminals and extragalactic AGN may be
surrounded by a wind near to the central engine. Theoretical analysis and
numerical simulation reveal considerable stabilization of relativistic jet flow
by a wind to helical and higher order asymmetric modes of jet distortion. When
velocities are measured in the source (inlet) frame, reduction in the absolute
velocity difference between jet and wind, $\Delta v = v_{jet} - v_{wnd}$,
provides stabilization in addition to stabilization provided by a high jet
Lorentz factor, but a high Lorentz factor wind is not needed to stabilize a
high Lorentz factor jet. However, the fundamental pinch mode is not similarly
affected and knots with spacing a few times the jet radius are anticipated to
develop in such flows. Thus, we identify a mechanism that can suppress large
scale asymmetric structures while allowing axisymmetric structures to develop.
Relativistic jets surrounded by outflowing winds will be more stable than if
surrounded by a stationary or backflowing external medium. Knotty structures
along a straight jet like that in 3C 175 could be triggered by pinching of an
initially low Mach number jet surrounded by a suitable wind. As the jet enters
the radio lobe, suppression of any surrounding outflow or backflow associated
with the high pressure lobe triggers exponential growth of helical twisting.
