Masses of the black holes in BBH systems produced by the primary stars (upper panels), and secondary stars (lower panels) as a function of the initial semi-major axis of the progenitor binaries. The 2D histograms show the number of BBHs per bin produced in the CHE10preMS models and the NoCHEpreMS models at metallicities Z = 0.001 and Z = 0.004. The marginal histograms show the distributions of the black hole masses for the CHE10preMS (black histograms) and the NoCHEpreMS (grey-filled histograms) models at Z = 0.001 and Z = 0.004.