  We propose a model for explaining the dependence in temperature of the Hall
effect of high Tc cuprates in the normal state in various materials. They all
show common features: a decrease of the Hall coefficient RH with temperature
and a universal law, when plotting RH(T)/RH(T0) versus T/T0, where T0 is
defined from experimental results. This behaviour is explained by using the
well known electronic band structure of the CuO2 plane, showing saddle points
at the energies ES in the directions (0,+/-pi) and (+/-pi,0). We remark that in
a magnetic field, for energies E>ES the carrier orbits are hole-like and for
E<ES they are electron-like, giving opposite contributions to RH. We are abble
to fit the experimental results for a wide range of hole doping, and to fit the
universal curve. For us kb*T0 is simply EF-ES, where EF is the Fermi level
varying with the doping.
