Histogram showing the intrinsic Hα/Hβ ratio for the different grids of models used in the classification procedure. Histograms are coloured following the usual convention for H II regions, PNe and shocks. The vertical dashed line represents the theoretical value of the Hα/Hβ ratio for Case B conditions and a temperature of 10⁴ K, typically assumed when recovering the extinction from the observed Hα/Hβ ratio in H II regions.