Comparison of ZAHB (continuous lines), MAHB (dotted lines), and TAHB loci (dashed lines) in the theoretical plane for three metallicities (Z = 0.00001, 0.00160 and 0.01570 in red, blue, and yellow, respectively) and three helium abundances (Y = 0.245, 0.295, and 0.345 in the top, middle, and bottom panels, respectively). In all displayed cases, M_MS = 0.900 M_⊙ and [α/Fe] = 0.3 have been assumed. All models have been shifted in log(L/L_⊙) and log T_eff such that the highest T_eff of each ZAHB locus aligns at the origin.