Probability, P(a_f < a_f,crit), that the dimensionless spin magnitude, a_f, of a merged BH is less than the value a_f,crit, as a function of a_f,crit. Each curve corresponds to a particular mass combination of the maximally spinning components of the merging BH–BH, as indicated in the legend. The probabilities are obtained out of 10⁵ trials of random and uncorrelated orientations of the spins of the BH components.