parameter, $\zeta_{\rm TiO/CaH}$, which quantifies the weakening of the strength of the TiO band (in the optical) as a function of metallicity. Subdwarfs are easily distinguished from dwarfs with solar abundances because they exhibit stronger metal-hydride absorption bands (FeH, CrH) and metal lines (Ca, Fe) as well as blue infrared colours caused by collision-induced H$_{2}$–H$_{2}$ absorption [@gizis97a; @lepine07c; @lodieu17a]. An independent classification scheme based on metallicity, gravity, and temperature has been proposed for M dwarfs by @jao08 and an extension to the L dwarf regime proposed by @kirkpatrick14 and @zhang17a.

In this paper, we present moderate-resolution 0.45–2.5 $\mu$m spectroscopy of 15 sdM, 16 esdM, and 12 usdM to infer their metallicities and T$_{\rm eff}$ by direct comparison with the latest BT-Settl synthetic models [@allard12]. In Section \[MplusTeff\_sdM:sample\] we present our sample of metal-poor M dwarfs drawn from the literature. In Section \[MplusTeff\_sdM:spec\_obs\] we describe our spectroscopic observations. In Section \[MplusTeff\_sdM:Models\] we introduce the BT-Settl models used to infer the physical parameters of metal-poor M dwarfs. In Section \[MplusTeff\_sdM:Comparison\] we infer the spectral type vs.metallicity/gravity/T$_{\rm eff}$ relation of the three M dwarf metal classes and compare it to independent but complementary studies as well as solar-type M dwarfs. In Section \[MplusTeff\_sdM:discussion\] 