Distribution of the properties of the population of BNS progenitors at their formation (ZAMS) and at three metallicities: Z = 9.5 × 10⁻⁵ (left), Z = 9.5 × 10⁻⁴ (centre), and Z = 1.4 × 10⁻² (right). From top to bottom, the three rows show the distributions of (1) primary masses, (2) secondary masses, and (3) initial semi-major axes. The standard evolutionary track is shown in blue, the equal-mass track in green, and the AIC track in orange. In black, we show the total population.