  The horizontal branch (HB) morphology of globular clusters (GCs) is mainly
described by metallicity. The fact that some clusters with almost the same
metallicity exhibit different HB demonstrates that other parameters are at
work. We present results from the analysis of the CMD of 72 GCs obtained with
the Advanced Camera for Survey (ACS) of the Hubble Space Telescope (HST). We
find a significant correlation between the HB color extension and the mass of
the hosting cluster, while the color distance between the HB and the red-giant
branch (RGB) depends on metallicity and age. We suggest that age and
metallicity are the main global parameters of the HB morphology in GCs, while
the HB extension is mainly due to internal helium variation, associated to
multiple stellar populations.
