It is argued that the origin of the buckling of the CuO2 planes in certain cuprates as well as the strong electron–phonon coupling of the B1g phonon is due to the electric field across the planes induced by atoms with different valence above and below. The magnitude of the electric field is deduced from new Raman results on YBa2Cu3O6+x and Bi2Sr2(Ca1−xYx)Cu2O8 with different O and Y doping, respectively. In the latter case it is shown that the symmetry breaking by replacing Ca partially by Y enhances the coupling by an order of magnitude, while the superconducting Tc drops to about two third of its original value.