Upper panel: distribution on the CMD of FG (red) and SG (blue) SGB targets. The SGB region where abundance change happens is indicated by the two green lines. Middle panel: abundance variation along the SGB. The y coordinate is the [C/Fe] difference between each target and the point of the MS anticorrelation (the magenta line in the inset of Figure 14) closest to that target. Bottom panel: [Fe/H] as a function of the position along the SGB. See text for more details.