[Ca/Mg] ratio as a function of the Ca abundances for several stellar populations, adapted from Carretta et al. (2013b). Grey open circles are stars in eight dwarf spheroidal Milky Way satellite galaxies (Kirby et al. 2011); blue and red filled circles are RGB stars in NGC 2419 with [Mg/Fe] lower and larger than 0.0 dex, respectively, from Cohen & Kirby (2012) and Mucciarelli et al. (2012). Orange triangles are giants in ω Cen (Norris & Da Costa 1995), cyan circles are RGB stars in M 54 (Carretta et al. 2010b). Green squares are for giants in 22 Galactic GCs (Carretta et al. 2009b, 2010c, 2011, 2013a,b). Black symbols are stars in NGC 4833 from the present study (squares and circles for the UVES and GIRAFFE samples, respectively).