Examples of APOGEE spectra and the removal of the stellar contributions to the spectra. The top panel contains the continuum-normalized flux of 9 observed stellar sightlines (S/N 110; black lines), spanning 190 Å of the APOGEE wavelength range (range specified at the bottom of the figure). Overplotted in red are the best-fit synthetic spectra, and the blue shading indicates regions significantly affected by sky emission or other sources of uncertainty. The bottom panel contains these sightlines' model-normalized residuals R_λ (black lines), overplotted with the best-fit DIB profiles in green (Section 2.3). The shaded spans above have been masked. The gray shaded columns indicate regions affected by persistent stellar residuals. The spectra are ordered by Galactic longitude, and the impact of the rotation of the Galactic ISM can be seen in the shifting of the observed DIB central wavelengths (indicated with vertical green lines; Section 4.3). The horizontal black bar below the bottom residual shows the velocity scale.