Grid slices of the HMS-HMS grid at q = 0.60, 0.85, and 0.99 for Z = 10⁻⁴ Z_⊙. We do not show the grid below M₁ < 15 M_⊙ because such systems do not contribute to the BBH merger rate. The non-converged (or initial RLOF), unstable, and reverse stable mass transfer (see Briel et al. 2025, for reverse SMT models) models are marked with black dots, red diamonds, and pink circles, respectively. Models without mass transfer are marked with gray squares. SMT systems are marked with four different colored squares, where the exact color indicates a contact phase during the MS (gray squares), Case A (light blue), Case B (teal), or Case C mass transfer (dark green) as the first interaction of the binary. We show the ZAMS properties of BBH merger progenitors by encircling all of them with a black contour, based on a smoothed 2D histogram.