Differences between the absolute (Ab) PMs from Gaia DR2 and the PMs in the rest frame (RF) of the ONC based on the HST + Keck (HK). We subtracted the median values of the differences between the absolute and the relative PMs (﹩\widetilde{{\rm{\Delta }}{\mu }_{{\alpha }^{* }}}﹩, ﹩\widetilde{{\rm{\Delta }}{\mu }_{\delta }}﹩) = (1.6, 0.6) mas yr⁻¹ to take into account the bulk motion of the ONC. The inset panel is the same as the main panel, but the PM uncertainties of Gaia DR2 were increased by a factor of 3 (see Section 4.1.)