Density diagnostic diagram using the logarithm of the [O II] (3727+3729)/(7318+7319+7330+7331) and of the [S II] (4068+4076)/(6716+6731) line ratios. Each sequence of black squares, joined by the dashed black line, is a sequence of AGN photoionization models with constant power-law index (α = −1.5) and ionization parameter (U = 0.005), created by varying the electron density of the model in the interval n_e = 100 − 10⁶ cm⁻³ (from top left to bottom right) with a step Δlog₁₀n_e = 0.5. The three different sequences in the plot (from top right to bottom left) are associated with E(B−V) = 0, 0.5 and 1. Green circles represent the narrow components; the intermediate and very broad components are indicated by the golden and blue circle, respectively. Error bars for each point are estimated as described in the text.