Atlas of MS (black) and giants (red) spectra in the wavelength range 6750–6785 Å, used to define the γ₁ gravity-sensitive index. Lines with better-known parameters are indicated at the bottom with solid blue segments, while some lines with less certain parameters are indicated with dashed segments. Strongly gravity-dependent (low-g) lines are evident. Cyan- and green-colored wavelength ranges are used to sample respectively low-g and high-g lines in the γ₁ index definition.