Magnesium abundances for the samples of Nissen & Schuster (2010) and Carlos et al. (in prep.) in ID LTE, ID non-LTE, and 3D non-LTE. In all panels, the Fe abundance is based on a 3D LTE analysis of Fe II lines (Amarsi et al. 2019). The metal-rich end (−1 < [Fe/H]) of the α-poor population is further divided into two, α-poor 1 and 2, based on 3D non-LTE [Mg/Fe] ratios. Four metal-poor α-poor stars are marked with open squares, as they have slightly lower [Mg/Fe] in 3D non-LTE than the other α-poor stars at the same metallicity.