Galactic (lower panel) and M 31 (upper panels) globular clusters whose HB morphology can be classified from their CMD in the Mg2 vs. Hβ plane. The average 1σ error-bars are plotted in the upper part of the panels. The over-plotted grid of models is from the recent set by Thomas et al. (2011): continuous lines are iso-age loci of age 3, 5, 8, 12 and 15 Gyr as labeled in the lower panel; dotted lines are iso-metallicity loci for  [Z/H]  =  −2.25, –1.35, –0.33, 0.0 and +0.35, as labeled on top of the lower panel. The adopted models are for SSPs with [α/Fe] = +0.3. Blue squares are clusters classified as HB-blue, red-circles are clusters classified as HB-red. The main targets of this study are represented with open symbols and labeled. We also labeled three M 31 clusters lying above the age = 5 Gyr line (B010, B293 and B008) and two Galactic clusters lying on top or above the age = 8 Gyr line (NGC 5024 and NGC 3201). The arrows on the points for B337 and B008 show the position implied by the new estimates of Hβ (Galleti, priv. comm.).