A Double-Line Spectroscopic Binary (or Double-Lined Spectroscopic Binary) is a Spectroscopic Binary star system in which the (combined) spectrum shows a specific Spectral Line of some element twice, the variation in Wavelength implying a Doppler Shift indicating stars' relative Radial Velocity.
This is in contrast to a Single-Line Spectroscopic Binary (or Single-Lined Spectroscopic Binary), where some line varies, indicating the radial motion of an orbit around a companion, but no line is observed that clearly comes from the companion. This latter is identical to identification of Extra Solar Planets via the radial velocity method, the distinction being the smaller mass of a planet companion. A Spectral Resolution insufficiently precise to identify planets can still identify many Binary Stars by this method.
Stellar Parameter Determination