Number of mergers and characteristic masses at each merger generation. The blue histogram and the left-hand y-axis show the number of BBH merger events for each hierarchical merger generation, N_g. The orange (green) shaded area and the upper right-hand y-axis show the 25%–75% percentile of the chirp mass (primary mass) for merging BBHs of each generation. The orange (green) dots and lower right-hand y-axis show the average chirp mass (primary mass) for merging BBHs of each generation.