Nine representative spectral indices as a function of spectral type. Filled (red) circles: standard stars. Open (blue) circles: remaining target stars. Dashed (red) line: fit to straight line, parabola, or cubic polynomial, drawn only in the range of application of the fit. Top left and middle: TiO 2 and TiO 5 (Reid et al. 1995), with negative slopes and useable up to M7 V; top right: PC1 (Martín et al. 1996), a monotonous spectral indicator from K5 V to M8 V; centre left and middle: VO-7912 (Martín et al. 1999) and Color-M (Lépine et al. 2003), two indices very similar to the PC1. Note the logarithmic scale in Color-M; centre right: ratio B (Kirkpatrick et al. 1991), sensitive to several stellar parameters and, thus, with a large scatter in the spectral type relation; bottom left: CaH 3 (Reid et al. 1995), with a slightly larger scatter than pseudo-continuum or titanium oxide indices, due to metallicity; bottom middle: VO (Kirkpatrick et al. 1995), useable only for determining spectral types later than M4 V; bottom right: I2 (Martín & Kun 1996), with a linear range of variation from mid-K to mid-M and a sudden increase (or high dispersion) at late-M.