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