Comparison of the K-Mg anti-correlation in different GCs. Left panel: red and blue circles are the stars of the present work (same symbols as in Fig. 3) for NGC 4833 superimposed to stars in NGC 2419 from Cohen & Kirby (2012: dark grey circles) and Mucciarelli et al. (2012: light grey symbols). Middle panel: red circles are stars in NGC 2808 from Carretta (2015) and Mucciarelli et al. (2015). Right panel: red circles indicate stars in NGC 6715 (M 54) from the preliminary analysis by Carretta (in prep.).