A double-line spectroscopic binary (or double-lined spectroscopic binary, sometimes abbreviated SB2) is a spectroscopic binary star system (which consists of stars too close together to resolve separately) in which the (combined) spectrum shows a specific spectral line of some element twice, the two different wavelengths implying a difference in their Doppler shifts, indicating stars' relative radial velocity, which shows the cycle of movement characteristic of orbits. Observation shows a line which is sometimes single and sometimes shows two peaks, in a steady cycle, being single at the times in their orbit when there is no radial-velocity-difference between the two stars. The spectral class gives some indication of the mass of the visible star, and the ratio of masses is revealed by the relative radial velocities, giving a similar estimate of the smaller companion.
This is in contrast to a single-line spectroscopic binary (or single-lined spectroscopic binary, SB1), for which 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 case is identical to identification of extra-solar planets via the radial velocity method, the distinction being that the calculated mass of the companion is small enough to indicate a planet. A spectral resolution insufficiently precise to identify planets may still be able to identify many binary stars by this method.