Spectroscopic Binary is a type of binary star.

Another way to calculate star masses is to measure their relative velocities by moving their spectral lines via the Doppler. This is seen where the pair can not be resolved as a visible binary, but movement is used in the one-star spectra.

You don’t need to see two spectra, only one of the stars’ motion is required to deduce the presence of the binary system (why will one star be traveling alone?). Most binary stars are too close to separating the components, but Doppler shifts can deduce their presence.

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Typical velocities between binaries are 3 to 5 km/sec, so to detect this effect, very high-resolution Coude spectra needs to be taken.