Abundance ratios for light elements (Na, Al) and α‐elements (O, Mg), plotted against logarithmic iron‐to‐hydrogen ratio, [Fe/H]. The dashed line shows the solar ratios. The key to the symbols is given in the upper panel. The halo field star samples are from McWilliam et al. (1995; M95), Nissen & Schuster (1997; NS), Stephens (1999; S99), and the series of papers by Gratton (1989), Sneden et al. (1991b), and Gratton & Sneden (1988, 1991, 1994; GS). Disk stars are from Edvardsson et al. (1993; E93) and Nissen & Schuster (1997; NS). Element abundances for damped Lyα absorbers in the direction of several QSOs are taken from Vladilo (1998; V98) and Pettini et al. (2000; P00).