Stacked histograms of Lorentz factor estimates in XRBs (pink) and GRBs (blue), with different methods and data sources labelled. Lower limits for XRBs are marked with a pink triangle. With various caveats (see text), GRBs blast waves are ultra-relativistic, and at least some XRB jet ejecta are mildly or moderately relativistic. The banded pink and blue regions show the approximate regimes used to aid our discussion of XRB (⁠|$1.5\lesssim \Gamma _0 \lesssim 8$|⁠) and GRB (⁠|$\Gamma _0 \gtrsim 100$|⁠) blast waves, respectively.