Schematic of the jet model. The equilibrium boundary radius of the jet, R, is proportional to z^q, where z is the distance from the supermassive black hole. The parameter q < 1 depends on the pressure profile of the external medium that collimates the jet. We consider a cylindrical jet (q = 0), a “sausage-like” jet (q = 0.3), and a nearly parabolic jet (q = 0.4). In the sausage-like jet, the boundary oscillates about the equilibrium value. The emission originates from an ensemble of compact, off-axis emission features (yellow blobs) that propagate along the jet at the local flow velocity. The line of sight forms an angle θ_obs with the jet axis.