Predicted emission-line flux ratio of [Neii]12.81/[Neiii]15.55 (upper) and [Nii]121.7/[Niii]57.21 (lower) as a function of the density-sensitive flux ratio [Oiii]51.80/[Oiii]88.33. Solid and dotted lines denote U-constant and n-constant model sequences, respectively. Models with Z_gas = 0.2, 1.0, and 3.0 Z_⊙ are denoted by blue, green, and red lines, respectively. The models with Z_gas = 0.2 Z_⊙ or with n_H = 10^4.0 cm^-3 are not shown in the lower panel because [Nii]121.7 is extremely faint in those case. Observational data for M 82 and for the Antennae galaxy are shown in the lower panel.