Determining the mass of binary stars

[Mac Stewart]

During a lecture, it was mentioned that we can use the way binary starts orbit their center of mass to determine the masses of the two stars. When I think about this, it seems to me that this would only be useful in calculating their relative masses, and not their exact masses with respect to the earth, or the sun. Is this assumption correct, or there something I'm missing in the technique that allows the precise measure for the masses of the two stars?