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For intrinsically symmetric oppositely directed relativistic jets, their proper motions areThe ratio removes $D$ and yieldsTaking the product instead givesCausality requires $\beta \le 1$, so the limiting distance and inclination areHere and in e, “maximum” denotes the limiting $\beta \to 1$ bound; massive ejecta obey strict inequalities. The conversion is $1\mathrm{mas}\mathrm{da}{\mathrm{y}}^{-1}=5.6113\times 10^{-14}\mathrm{rad}{\mathrm{s}}^{-1}$, henceBoth bounds occur together in the light-speed limit. At any specified smaller distance, ${\beta }^2=x^2+(1-x^2)(D/D_{\max }{)}^2$ determines the intrinsic speed.