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, , 445, 433) Extended model atmospheres agree well in the 6200 -- 7400 Angstroms 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, in press) distance scale.
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Gizis et al. (1997) studied this question.