Thermal equilibrium state of the diffuse interstellar gas obtained with the Paris-Durham code and displayed in a particle density versus thermal pressure diagram. The color code highlights the three branches of the equilibrium state: the dynamically stable WNM (red) and CNM (blue) phases at ~10⁴ K and ~80 K, respectively, and the dynamically unstable branch (green) at intermediate temperatures. The light blue and red zones show the regions of the diagram where the cooling rate is larger or smaller, respectively, than the heating rate. Light gray lines are isothermal contours from 1 to 10⁶ K (from bottom right to top left).