Variation of chemical elements from ¹H to ¹⁸O in five stages along the RGB phase of a selected star with initial mass 0.78 M_⊙ and metallicity Z = 0.0002. The HRD of the corresponding star begins from the zero-age main sequence, with the five marked stages. Each element is indicated by the coloured lines. The cyan-shaded area displays the thermohaline instability region at each evolutionary stage (bottom-right panel), the grey-dotted area indicates the H-burning shell. All first five panels share the same scale on the x- and y-axis.