  We study the FeH Wing-Ford band at 9850 - 10200 Angstrons by means of the fit
of synthetic spectra to the observations of M stars, employing recent model
atmospheres. On the basis of the spectrum synthesis, we analyze the dependence
of the band upon atmospheric parameters. FeH lines are a very sensitive surface
gravity indicator, being stronger in dwarfs. They are also sensitive to
metallicity (Allard & Hauschildt 1995). The blending with CN lines, which are
stronger in giants, does not affect the response of the Wing-Ford band to
surface gravity at low resolution (or high velocity dispersions) because CN
lines, which are spread all along the spectrum, are smeared out at convolutions
of FWHM $\simgreat$ 3 Angstrons. We conclude that the Wing-Ford band is a
suitable dwarf/giant indicator for the study of composite stellar populations.
