Carbon-oxygen core mass (M_CO) versus the central ¹²C mass fraction abundance (X_C12) at the end of central He burning phase. The different colors identify the different sets of models (see side legend) where RAU is Rauscher et al. (2002); PGN is Pignatari et al. (2016); SIE is Sieverding et al. (2018); RIT is Ritter et al. (2018a); L&C is Limongi & Chieffi (2018); BRI is Brinkman et al. (2019, 2021); LIM is Limongi et al. (2024); RLC is Roberti et al. (2024a); ROB is Roberti et al. (2024b); and KEP is Jeena et al. (2024). Different symbols identify different initial masses (see top legend). Models with and without a C-O shell merger are represented by filled and empty symbols, respectively. The two gray shaded areas represent the covariant confidence ellipses relative to the models with C-O mergers within one and two standard deviations, σ, centred at M_CO = 4.02 and X_C12 = 0.176.