EW(Ti i) index measured in two grids of synthetic spectra based on KURUCZ (left) and MARCS (right) atmospheric models. The x-axis shows effective temperature (with inverted scale to ease the comparison with the SpT sequence), the colours indicate metallicity [Z] and the shapes indicate surface gravity (circles are log  g = 0.0 and squares log  g = 1.0 dex) of each synthetic spectrum from the grid. The vertical bar represents the median error in EW(Ti i) measurements.