Granular materials exhibit a behaviour that is strongly dependent on density, effective confining pressure and grain characteristics. The complexity is such that, in the past, for a given sand, loose and dense specimens, or specimens subjected to very low or very high confining pressures were considered as different materials and were modelled as such, using different constitutive parameters. The introduction of the state parameter improved both understanding of the behaviour and modelling of granular materials. Several alternative definitions have been proposed for the state parameter but they suffer from some limitations, especially when sand is very loose, has fines, or is subjected to high confining pressures. The purpose of this paper is to describe those deficiencies and to introduce a new state index based on a novel reference state curve. The proposed reference state curve is then applied to understanding and predicting some important aspects of soil behaviour such as: (a) stress ratios at the peak point of undrained effective stress paths and (b) the ‘reverse behaviour’ of sandy soils that show an unusual behaviour in undrained shearing under high effective confining pressure.