[en] Evidence for d-wave superconductivity from measurements of the local conductivity is rather indirect and not very compelling. Very recently, experimental data on the surface resistance taken at five different frequencies on very high quality single crystals of YBa₂Cu₃O₇ have become available. Having the frequency as additional variable provides a much more stringent test for the predictive powers of the d-wave model. Because of the high quality of the samples the authors expect that at low temperatures the nonlocal limit applies to the surface resistance. A nonlocal relation between current and field would allow a direct investigation of the order parameter anisotropy by varying the wave vector q of the radiation relative to the crystal axes