Asteroseismic diagram showing the run of (⟨ rr₀₁ ⟩ + ⟨ rr₁₀ ⟩ ) / 2 as a function of ⟨ Δν ⟩ for stars with masses in the range 0.9−1.3 M_⊙ during the MS. Models have been calculated with an initial helium abundance Y = 0.275, solar metallicity Z/X = 0.0245, and mixing-length parameter α_conv = 0.60. Evolutionary stages with decreasing central hydrogen abundance X_c are pinpointed.