Recalibration of the ζ_TiO/CaH parameter (denoted as ζ for short) based on a sample of 83,213 M-dwarf spectra from the LAMOST regular survey. ζ is a combination of the TiO5, CaH2, and CaH3 spectral indices (see definition in Equation (5)) and has been shown to be correlated with the metallicity. The black curve is an iso-ζ line with ζ = 1, providing an estimate of the TiO-to-CaH ratio in an object of a solar metallicity. Smaller ζ values correspond to stars of lower metallicities. The index was first introduced in Lépine et al. (2007) and then recalibrated in Dhital et al. (2012) and Lépine et al. (2013). Table 2 compares these calibrations.