Schematic diagram qualitatively illustrating the expected relation between T_e(OIII) and T_e(OII) at fixed Z_Te in different regions of the T_e(OIII)–T_e(OII) plane. Grey lines illustrate arbitrary tracks of constant Z_Te. Regions A and B denote the approximate regimes where Z_Te is likely dominated by only one ionisation state of oxygen (O⁺⁺/H⁺ or O⁺/H⁺, respectively). Region C denotes the approximate regime where both ionisation states are likely significant, and a uniform anti-correlation between T_e(OIII) and T_e(OII) at fixed Z_Te is expected. We stress that the simplified picture illustrated here may change if, for example, the nebulae are not radiation bounded, or if the density distribution in the O⁺⁺ and O⁺ zones are very different.