Distributions of GCs on the slope g(ix) (spectral absorption feature index ∼ binary fraction) and [Fe/H] plane. Blue dashed and red solid lines are the average slope () and 0. The red point is for NGC 6121, the crosses are for NGC 1851, 3201, and 7089 (not used in the calculations of and RMS for the sake of large deviation from the average values of all GCs ), and the green point+crosses are for the other GCs. Comparing with , they are of similar magnitude except Fe5015 (for which the EWs from the U17 and S05 data are very different; see the right panel of Fig. 1. For the above the reason, we do not consider this index as the binary fraction-sensitive index). Because we have not the slope errors, we use the index value of NGC 6121 as the error in the RMS calculations.