[S II] λλ 6717/6731 intensity ratio vs. Hα/[S II] ratio. Circles refer to intensity ratios in the narrow wings of [S II] and Hα, when present (violet: approaching component; purple: receding component). Other symbols are as in Fig. 12. Solid lines/polygons indicate SNR, H II, and PNe loci as in Fig. 40. Dashed horizontal lines enclose the range where the [S II] 6717/6731 ratio is a useful measure of density (increasing from top to bottom).