Illustration of the problem, showing Fe and Mg abundances for stars in the solar neighbourhood. a) Based on data by Fuhrmann (see text for references). At each value of [Fe/H] the stars fall into two groups with distinctly different [Mg/Fe]. b) Based on data for stars with halo velocities from Nissen & Schuster (2010). The two lines, drawn by hand, illustrate the separation in high- and low-α stars identified by Nissen & Schuster (2010). c) Illustration of the generic problem treated here.