dex. The difference in gravity between field M dwarfs and M subdwarfs is below our error bar of 0.5 dex, while the difference between metallicities is five times smaller than our error bars.

![Gravity as a function of spectral type for sdM (red dots), esdM (green triangles) and usdM (blue squares). Error bars on spectral type and gravity are $\pm$0.5 and $\pm$0.5 dex, respectively. The point for the sdM7 subdwarf corresponds to LHS377 from @rajpurohit16a. []{data-label="fig_MplusTeff_sdM:plot_SpT_logg"}](plot_subdwarfs_SpT_logg.png){width="\linewidth"}

![Metallicity as a function of spectral type for sdM (red dots), esdM (green triangles) and usdM (blue squares). Error bars on spectral type and metallicity are $\pm$0.5 and $\pm$0.5 dex, respectively. []{data-label="fig_MplusTeff_sdM:plot_SpT_FeH"}](plot_subdwarfs_SpT_FeH.png){width="\linewidth"}

Metallicity scale {#MplusTeff_sdM:Comparison_MH}
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Our work is extending the current metallicity scale of M dwarfs to metallicities below $-$0.5 dex with an accuracy on the calibration of the order of 0.25–0.5 dex. The classification is available in the optical for subdwarfs [@gizis97a; @lepine07c; @jao08; @savcheva14; @kesseli17a] but this is the first time this is attempted for a large sample of M0–M9 dwarfs [see also @rajpurohit16a]. The determination of the metallicity scale of subdwarfs is key for several reasons. First, the range of metallicities for M subdwarfs is poorly constrained up to now 