Temperature classification of the M-type giants is based primarily on the increasing strength of the TiO bands. At K7, the TiO 5448 bandhead can just barely be detected; after M3 the strongest bands become saturated, and the temperature type is assigned on the basis of the strength of the fainter bands. To avoid complications due to abundance or population effects or the "veiling" phenomenon in which the spectral features appear washed out (this can be a particular problem in the variable M-type stars, such as the Mira variable stars), it is best to judge the spectral type on the basis of the ratios of the strengths of these bands.
The metallic lines reach a maximum strength in the early M-type stars; in later types the successively stronger absorption due to TiO begins to reduce the line strengths in the spectral region illustrated here. The Ca I 4227 line, however, continues to increase in strength in the giants, especially after M5.
This figure uses 3.6 Å resolution flux spectra.