Two-dimensional histograms of the masses of black holes in merging BBH systems, for the same GALSEVN models as in Fig. 11, for the metallicities Z = 10⁻¹¹, Z = 10⁻⁶, Z = 0.0001, and Z = 0.001 (in different panels, as indicated). The quantity m₁ refers to the mass of the BH formed from the primary star, whereas m₂ refers to that of the BH formed from the secondary star. The total number of BBH mergers in the modeled stellar population is indicated in the lower-right corner of each panel. The identity relation is shown as a dashed line.