We present a spectroscopic classification system for M-dwarfs and M-subdwarfs based on quantitative measures of TiO and CaH features in the region \lambda \lambda 6200 - 7400 \AA .
Our sample of cool stars covers the range from solar metallicity stars to the most extreme subdwarfs known .
Using synthetic spectra computed by Allard and Hauschildt ( 1995 ) , we derive metallicities for the stars .
Stars are classified as dwarfs ( M V ) , subdwarfs ( sdM ) , or extreme subdwarfs ( esdM ) .
These classifications correspond to [ m / H ] \approx 0.0 , -1.2 , and -2.0 respectively .
Our metallicity scale agrees with theoretical HR diagrams and HST globular cluster measurements .
We discuss some nearby subdwarfs of particular interest in light of our metallicity scale .