Example of an incorrect isochrone (dashed black line), as obtained when not accounting for the different morphologies of the post-RGB phase of the two tracks used in the interpolation. The isochrone has an age of 14 Gyr. The two tracks used for the interpolation of the post-RGB phases are shown in blue and orange. The initial metallicity is Z = 0.002, and the initial He mass fraction is 0.249. The two tracks shown are reduced tracks obtained following the method described in Section C.1