Magnesium abundances for HD 84937 (left) and HD 140283 (right) based on line-by-line equivalent widths. Error bars reflect uncertainties in T_eff, log g, [Fe/H], and the ξ adopted in the 1D analysis, as well as a 5% uncertainty in the equivalent width (10% uncertainty for the weak Mg I 473.0 nm line in HD 84937 and for the blended Mg II 448.1 nm line). Weighted lines of best fit for Mg I are shown, excluding the saturated 517 nm triplet and the 457.1 nm line that forms in the upper layers of the photosphere. The shaded horizontal area represents the 3D non-LTE result and its uncertainty from the Mg II 448.1 nm line. The weighted mean abundances, the abundance difference, and their uncertainties indicated in the legend were calculated using the framework from Ji et al. (2020).