2-dimensional contour plot of c₂ against R_V for c₃ = 0.0 (top) and c₃ = 1.0 (bottom), and c₄ = 0.4 in both cases. In both panels, the inner (black), middle (red) and outer (green) contours correspond to 68.3%, 90% and 99% confidence intervals, respectively. The best-fit c₂ and R_V values are indicated by a black cross, and the corresponding [R_V,c₂] pairs for the mean SMC, LMC and Milky Way (MW) extinction curve are also shown.