New HB morphology index, τ_{HB, 12 Gyr}, as a function of cluster iron abundance (Carretta et al. 2009). The two lines display synthetic HB models at fixed cluster age (12 Gyr), but either with a canonical helium content (∂Y = 0, blue line) or with an internal spread in He of ∂Y = 0.03 (black line). The coloured triangles identify three models for [Fe/H] = −1.6 for three different combinations of ∂Y and ΔM (see legend). Red squares mark the eight second parameter clusters. The error bar on the left corner gives the 0.1 dex error on the metal content.