SDSS median stacked spectra for the sample of massive and passive ETGs. Left and right upper panels show, respectively, median stacked spectra with a fixed redshift (0.15 ≲ z ≲ 0.19) and different masses (with mass increasing from blue to red) and with a fixed mass (11.25 < log (M/M_⊙) < 11.5) and four different redshifts (0.04, 0.08, 0.16, 0.23) (with redshift increasing from blue to red). Lower panels illustrate the fractional differences (defined as (f_i−f_REF/f_REF) × 100, where f_i is the flux of the ith spectrum and f_REF is the reference flux) among the stacked spectra as a function of wavelength. In particular, to show the redshift and the mass dependence we used as reference, respectively, the median stacked spectrum obtained for 11.25 < log (M/M_⊙) < 11.5 and 0.07 < z < 0.09, and the one corresponding to 10.75 < log (M/M_⊙) < 11 and 0.15 < z < 0.17. Vertical grey lines mark some of the best-known absorption lines.