Top: Evolution of kinetic gas temperature, T_k (blue solid line), radiation temperature, T_γ (dashed green line), and 21 cm spin temperature, T_s (orange dash-dotted line), as a function of redshift. Bottom: Evolution of the global 21 cm differential brightness temperature as a function of redshift. At z_dec,1% (T_k = 99% T_γ), thermal decoupling starts to be effective. At z(δT_b,min), δT_b reaches its lowest point, corresponding to the maximum absorption during the dark ages.