Diagnostic diagram plotting log [O III] λ5007/Hβ vs. log [O I] λ6300/Hα. The symbols have the following meanings: open triangles, disk H II regions, filled circles, early‐type (S0–Sbc) H II nuclei; open circles, late‐type (Sc–I0) H II nuclei; asterisks, starburst nuclei. The curves denote the theoretical models of disk H II regions described in the caption of Fig. 7.