[en] Superconductivity in the cuprates is believed to be controlled by the density of mobile charges in the CuO₂ planes. In particular, the charge transfer process from the chains to the planes seems to play an important role. Consequently, it is crucial to observe directly the influence of different types of perturbations on the electronic structure of these compounds. The crystal field (CF) spectroscopy of the rare earth allows us to make these observations, since in the perovskite-type compounds YBa₂Cu₃O_x(123) and YBa₂Cu₄O₈(1248) the replacement of the Y ions by most of the magnetic rare-earth (R) ions does not have a detrimental effect on the superconductivity. The (2J+1)-fold degeneracy of the ground-state J-multiplet of the R ions will be partially lifted under the action of the CF potential created by the neighbouring atoms. By means of inelastic neutron scattering experiments it is possible to observe directly the transitions between the CF states. This means that we can obtain useful information on both the structural and the charge distribution parameters in the vicinity of the R ion. In the 123 and 1248 systems, the R ions are sandwiched between two CuO₂ planes, thus the CF interaction at the R sites constitutes an ideal probe of the local symmetry and charge distribution of the superconducting planes. In the first part of this work, we discuss the importance of the intermediate coupling and J mixing effects on the determination of the CF parameters of the 123 compounds. In order to quantify the charge transfer process from the chains to the planes, we performed a detailed analysis of the CF of Er³⁺ in the 123 and 248 compounds under the following conditions: oxygen deficiency, Zn and Ni doping of the Cu sites, external pressure and fast neutron irradiation. In parallel, we present conclusions obtained from diffraction experiments. (author) figs., tabs., 113 refs