A mathematical relationship between the masses of the components in a spectroscopic binary and the orbital inclination. It is the only information about the masses that can be derived when the spectrum of just one star is visible. If that star is of mass m1 (in solar units), then the mass function is given by (m2 sin i)3/(m1+m2)3, where m2 is the mass of the unseen star and i is the inclination of the orbit.