This figure illustrates a small part of the absorption spectrum for a spectroscopic binary, HD 80715. The period is a little less than four days. The Doppler effect splits the absorption dip near 6445 Angstroms into two lines with periodically varying separation.
In the above spectrum, the single strong line at 0.061 days is the sum of contributions from the two stars when they are moving transverse to the line of sight (no radial velocity). From 0.334 to 1.886 days the line is split, with the red-shifted component somewhat weaker than the blue-shifted component. At 1.886 days the two lines merge again, and from 2.148 to 3.677 days the blue-shifted component is somewhat stronger than the red-shifted one. This spectrum suggests that the star moving away from us from 0 up to ~ 1.9 days (and toward us in the second half of the period from ~ 1.9 days up to ~ 3.8 days) has a somewhat stronger absorption line than the other star.