Normalized distribution of the spin of the second-born BH as predicted by our model for GWs observable at aLIGO design sensitivity, in light blue, vs. the underlying population of merging BBHs, in gray. For comparison, we show the same distribution obtained with MESA simulations of BH–He-star binary systems with M_BH = 30 M_⊙, M_He-star = 35 M_⊙ and metallicities Z_⊙ (orange), 10⁻¹ Z_⊙ (green) and 10⁻² Z_⊙ (red) under the assumption that the initial separation is log-uniform in 5 <  A/R_⊙ <  63.