  Band structure trends in hole-doped cuprates and correlations with T_c are
interpreted within the s--d exchange mechanism of high-T_c superconductivity.
The dependence of T_c on the position of the copper 4s level finds a natural
explanation in the generic Cu 3d, Cu 4s, O 2p_x and O 2p_y four-band model. The
Cu 3d--Cu 4s intra-atomic exchange interaction is incorporated in the standard
BCS scheme. This dependence of T_c in the whole interval of 25--125 K has no
alternative explanation at present, and possibly this quarter of a century
standing puzzle is already solved.
