XSL (‘PC’: PARSEC/COLIBRI; ‘P00’: Padova00), E-MILES P00, Maraston et al. (2009, M09), and Conroy et al. (2018, C18) SSP model spectra of 1 Gyr (panel a) and 10 Gyr (panel b) solar-metallicity stellar populations. The spectra are smoothed to R = 500. The M09 spectra are displayed at their original resolution (R ≈ 500). All spectra are normalised to a common I-band flux. The residual of the XSL P00 model from the E-MILES P00 model is shown in grey in each panel.