Sample electron densities. Top: histogram of the [S II]λ6731/16 ratio for H II (solid blue lines) and DIG (dashed orange lines). The area hatched with backslashes represents non-physical values, and the forward slash covers the range of Ne that is fairly invariant with T_e. Bottom: [S II]λ6731/16 emissivity grid for 3kK ≤ T_e ≤ 20 kK and 1 ≤ log₁₀ N_e(cm⁻³) ≤ 7. Contours of [S II]λ6731/16 are overplotted in white, and the corresponding value is shown inline.