Thermodynamic structure of the stellar models of different mass at the age of 12 Gyr, in the density - temperature plane. The boundaries of the partial helium and partial hydrogen ionisation regions are shown, as well as the region in which corrections to the ideal gas EOS become important, limited at high density by the pressure ionisation boundary. The top panel refers to 1G models; the bottom panel to 2G. The same structures highlighted in Fig. 2 are highlighted here. Below 0.08 M_⊙ the mass step is 0.001 M_⊙. This allows one to see how fast in mass steps the transition between stars and brown dwarfs is.