The S-matrix formalism gives a model independent description of the Z lineshape. Determined from LEP I data alone the error on the gamma-Z interference is large and its contribution to the hadronic cross-section is strongly correlated with the Z mass. From the measurements of fermion-pair cross-sections and asymmetries at LEP II the gamma-Z interference can be determined. As a consequence the error on the Z mass is drastically reduced and only slightly larger than the error resulting from the Standard Model fit.