Comparison of emission-line ratios and velocity dispersion for supernova remnants (SNRs) and H II regions. Panel (a): Line ratios of [S II] versus velocity dispersion of SNRs and H II regions. The top axis shows the FWHM. The Hα velocity dispersion changes with the [S II]λ6716/[S II]λ6730 ratio for SNRs (red dots) and H II regions (in grayscale). The ratios above 1.4 are not physical and are due to noise. The horizontal dash-dotted lines indicate different electron densities with the corresponding value next to it. Panel (b): Velocity dispersion of Hα changes with [S II]λ6716,6730/Hα ratio for SNRs (red dots) and H II regions (in grayscale). For SNRs that overlap with a H II region, the symbol is an empty circle in light red. These two criteria work effectively together in distinguishing between SNRs and H II regions.