We analyse the saturation pressure (SP) behaviour along the coexistence curve liquid-gas in reduced variables, containing only the Zeno line parameters. Using experimental and simulation data for substances (Ar, N-2, Hg, Cs) and two models (van der Waals and Lennard-Jones) we create a global 3D picture of SP dependence on temperature and density. We also show that the well-known empirical Timmermans relation and partly Riedel equation are the consequences of the van der Waals equation and the rectilinear diameter law. (C) 2013 Elsevier B.V. All rights reserved.