We report finite-cluster diagonalizations which clarify the role of apical oxygen orbitals and of the copper orbitals in the ground-state electronic properties of high-temperature superconductors. The hole density distribution and spin - spin correlation functions are analysed as functions of interoxygen hopping and other parameters. The main conclusion is that for universally accepted parameters for planes, small levels of doping and no phonons, the three-band Hamiltonian suffices to describe the properties of the ground state and the inverse photoemission.