Examples for He I IR line and neighbouring spectral region as it changes along with a spectral type similar to Fig. 2 while also displaying Voigt fits. The observed spectrum is denoted in blue, fit in red, and fit component of the He I IR line in green (dashed). Vertical grey dashed lines mark the position of lines considered in the fit (Si I line at 10 830.057 Å, He I IR line at 10 833.25 Å, two unidentified lines at 10 831.9 and 10 832.3 Å.) Top left: M0.5 V star J02222+478/BD+47 612. Top right: M1.0 V star J00051+457/GJ 2. Middle left: M1.5 V star J02123+035/BD+02 348. Middle right: M2.0 V star J01013+613/GJ 47. Bottom left: M2.5 V star J00389+306/Wolf 1056. Bottom right: M3.0 V star J02015+637/G 244-047.