Luminosity ratio between the [C II] luminosity arising from the ionized medium only, [C II], and the [N II]_205 μm line luminosity, as a function of the ionized gas density, n. Colored squares mark the results of our Cloudy models, color-coded by the gas metallicity. The models were performed on a grid of 0.25 dex spaced in log n, and slightly horizontally displaced in the plot for the sake of clarity. We also show our analytical predictions for the same line ratio, color-coded by the gas temperature, T_gas. Dotted, short-dashed, long-dashed, and dot-dashed lines mark models at increasing gas metallicity. The predicted line ratio is unaltered by T_gas, and is only slightly dependent on n, whereas it is linearly dependent on the relative abundance of carbon and nitrogen ions, [C⁺/N⁺].