Example of a two-dimensional slice of the four-dimensional grid showing their initial BH–He-star orbital period p_i (in days) and initial He-star mass m_{He − star} (in M_⊙) for log₁₀(Z) ≃  − 3 and m_BH1 = 29.39 M_⊙. The final mass m_BH2 and spin a₂ of the second-born BH, as well as merger timescale T_merger, are coloured according to the legend of each panel. All successful MESA simulation stopped at carbon depletion (square markers) while other termination flags are shown in the bottom legend. The merger timescale colour bar is capped at 14 Gyr.