Temperature dependence of the viscosity η(T) of fragile and strong polymer and non-polymer glass formers was analyzed. A temperature T_x was introduced which marks the boundary between a high- and low-temperature regime, where different approaches are applied to interpolate η=η(T). At T<T_x a master curve is obtained for polymer and non-polymer glass formers by scaling the quantity (T_g/T−1) with a factor F=T_x/(T_x−T_g) where T_g is the glass transition temperature. F is related to the steepness parameter m via F≅0.079m. A continuous transition from the master curve of non-polymer systems to the one of polymers is found if molecular weight is systematically increased. T_x is found to be close to the critical temperature T_c reported from analyses of the dynamic susceptibility by mode coupling theory. It is tentatively concluded that the magnitude of the interval between T_x and T_g defines the degree of fragility. At T>T_x deviations from the master curves occur, and the Vogel–Fulcher–Tammann equation provides a very good fit to highest temperatures. The two master curves at T<T_x can be interpolated by several formulae.