extreme subdwarfs, and ultra-subdwarfs, respectively. Metal-poor M dwarfs tend to be warmer by about 200$\pm$100K and exhibit higher gravity than their solar-metallicity counterparts. We derive abundances of several elements (Fe, Na, K, Ca, Ti) for our sample but cannot describe their atmospheres with a single metallicity parameter. Our metallicity scale expands the current scales available for midly metal-poor planet-host low-mass stars. Our compendium of moderate-resolution spectra covering the 0.45–2.5 micron range represents an important legacy value for large-scale surveys and space missions to come. ]{}

Introduction {#SpClass_sdM:intro}
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Mass, gravity, temperature, and metallicity constitute key parameters to understand the formation and evolution of any type of stars. M dwarfs represent the largest community among members of the solar neighbourhood [@henry06; @bochanski10; @kirkpatrick12] and have also become attractive targets to search for Earth-like planets [e.g. @nutzman08; @quirrenbach12a; @sozzetti14]. While the metal content of solar-type stars can be measured with great accuracies [e.g. @adibekyan12a], the metallicity of M dwarfs is more difficult to ascertain due to the significant amount of absorption bands [@kirkpatrick99]. Several groups conducted complementary surveys to assess the metallicity of M dwarfs based on photometry and colours [@bonfils05a; @johnson09a; @schlaufman10a; @neves12; @hejazi15; @dittmann16] and high-resolution spectra or spectral synthesis 