  We present spectra of three M subdwarfs which are common proper motion
companions to F or G subdwarfs of known metallicity. The assumption that the
companions have the same composition allows us to test the Gizis (1997, AJ,
113, 806) M subdwarf classification system and its correspondence to
metallicity. The results are in excellent agreement with the Gizis (1997)
scale, thereby showing that the Allard & Hauschildt (1995, ApJ, 445, 433)
Extended model atmospheres agree well in the 6200 -- 7400 Angstrom region for
cool metal-poor stars. We also show that the results are consistent with the
main sequences of globular clusters using the Reid (1997, AJ, 114, 161)
distance scale.
