Illustration of the selection boxes adopted for the computation of the SMI. Left panel: CMD of the Galactic globular cluster NGC6229, reported to the mean distance of M 31. Right panel: CMD of the Sample A globular cluster MCGC5. In each panel, the vertical segments mark, from left to right, the boundary between the blue (B) and red (R) part of the HB and the red limit of the HB selection. The continuous horizontal line is the mean level of the HB, while the dotted horizontal segments enclose the stars selected as bona-fide HB. In both diagrams Blue and Red HB stars selected to compute SMI are plotted in dark and light grey, respectively.