Calibration of Zhang et al. (2018) indices using our data set. Top panels: calibration of the Zhang et al. (2018) reddening-free SpT indices with the SpT derived in Sect. 3.1. Young, mid-gravity, and field objects are shown in orange, blue, and gray, respectively. The three available calibration curves are shown as solid (full sample), dashed (young sample), and dash-dotted (field sample) black lines. The shaded gray region is the original range of application. Lower panels: SpT derived for each index versus the SpT derived from the templates. The solid black line shows the 1:1 linear relationship, while the dashed lines mark a difference of one SpT.