Spectroscopic binary fractions and respective uncertainties of massive star clusters as a function of primary stellar masses. Horizontal bars indicate the approximate mass ranges probed by the respective studies estimated based on the indicated spectral types, see Sect. 7.2 for references. The grey-scale color gradient shows approximate cluster ages. For comparison, the multiplicity statistics for different spectral types and masses from Offner et al. (2023) are shown as crosses (see their Table 1).