Curvature, α, as a function of the large frequency separation Δν for RGB stars of solar metallicity, colour-coded by mass. In each panel, the mass that is indicated in the legend is shown on top of all other masses. The red solid and yellow dashed lines are fits to all models of that mass. For the solid line both the slope and the intercept were free parameters, while for the dashed line the slope was kept fixed to the slope of the fit to all models.