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 [Nii]121.7/[Nii]205.4. Solid and dotted lines denote U-constant and n_H-constant model sequences, respectively. Models with Z_gas = 1.0 Z_⊙ and 3.0 Z_⊙ are denoted by green and red lines, respectively. Only the models with n_H ≤ 10² cm^-3 are plotted for Z_gas = 1.0 Z_⊙, and models with n_H ≤ 10³ cm^-3 are plotted for Z_gas = 3.0 Z_⊙. This is because the [Nii] lines are faint in clouds with a high density owing to the collisional de-excitation effects.