Upper-left panel: evolutionary tracks of selected stellar models (no convective core overshooting during the H-burning phase) with Z = 10^-5, Y = 0.245, and M/M_⊙ = 0.5,0.6,0.8,1.0,1.2,1.4,1.6,1.8,2.0,2.2,2.4,2.6,2.8,3.0,4.0,5.0,6.0, and a scaled solar heavy elements mixture. All tracks for low-mass stars (apart from those masses with lifetimes much longer than the Hubble time) are displayed up to the tip of the red giant branch (RGB). Upper-right panel: selected isochrones for the same chemical composition, with ages t(Gyr) = 0.05,0.10,0.20,0.5,1.0,2.0,4.0,6.0,8.0,10.0,12.0,14.0. Lower-left panel: as in the upper-left panel, but for Z = 0.05, Y = 0.316. Masses equal to M/M_⊙:7,8,9,10 are also shown. Lower-right panel: as in the upper-right panel, but for the metal-rich chemical composition.