Comparison of the behaviour of the Mgb, CaHK, Ca4455, [MgFe], Fe5015, and NaD absorption-line indices as a function of the Hβ index. The shaded areas represent XSL PARSEC/COLIBRI models, with spectral resolution varying from σ = 13 km s⁻¹ (the XSL native resolution) to σ = 60 km s⁻¹ (the minimum E-MILES resolution in the optical). Black lines represent E-MILES BaSTI model predictions, with dotted, dashed, and solid lines representing [Fe/H] = +0.2, 0.0, and −0.4 dex, respectively, measured at the original E-MILES resolution. The solid blue line represents the predictions of the C18 solar-metallicity models.