1997PASP..109.1233G

begin{tex}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 – 7400Åregion 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.