D10) and doesn’t show any obvious correlation. Also results by G10 agree on the fact that age is the second main parameter driving the HB morphology.\
It is also worth noticing that most of these results are based on optical CMDs and HB morphology parameters (like the well known HBR from Lee et al. 1994), which tend to minimize the importance of blue tails. On the contrary using proper combinations of Ultra-Violet (UV) and optical filters has an important impact both in term of HB classification and comparison with theoretical models.

Still, age is not able to explain exhaustively the HB morphology. Detailed cluster to cluster comparisons have shown that there are systems with similar iron content and age, but remarkably different HB morphologies. A clear example is given by the three clusters M3 - M13 - M80, as shown by Ferraro et al. (1997, 1998) and at present there is hardly a scenario able to give a satisfactory explanation for their different morphologies.\
As suggested by Buonanno et al. (1985) and Fusi Pecci et al. (1993), age might be one of many and probably the most important HB second-parameter, but not the only one. Buonanno et al. (1995) argued 