Panel a: Na–O anti-correlation for all globular clusters from Carretta et al. (2009) in grey dots superimposed by the 11 NGC 3201 stars in common with our sample with available [Na/Fe] and [O/Fe]; symbols are the same as in Fig. 2. Smooth spline function is shown as red dashed line to highlight the anti-correlation in our data. Peculiar stars indicated by green crosses are not considered in the fitted function based on the selection of panel d. Panel b: Correlation between sodium and cyanogen, which is a proxy for nitrogen abundances for the 18 stars in common with available [Na/Fe]. A straight line is fitted to S1 stars. Panel c: Same as (b) but for CH and oxygen for stars in common with available [O/Fe]. The fitted line is shifted by 0.1 mag in δG4300 to match the only S2 star. Panel d: Distribution of CN and CH indices for all 28 NGC 3201 good quality member stars, revealing three groups of stars: sequence 1 (S1), sequence 2 (S2), and peculiar (pec). A second-order polynomial was fitted to S1 stars and the same curve was shifted by 0.1 mag in δG4300 as in panel c to match the S2 stars, and by − 0.1 mag to match the peculiar stars. Mean uncertainties are shown by the cross on the upper right corner of the panel.