Example of a possible route leading to the formation of a BH-BH merger similar to GW170104. This example follows the classical isolated binary evolution channel. In this model (M20.B) we employ the Geneva BH natal spins, assume that massive BHs do not receive natal kicks and that their spins are aligned with the binary angular momentum (Θ₁ = Θ₂ = 0°), producing an upper limit on the effective spin parameter (χ_eff). Yet, this system has χ_eff = 0.09, within LIGO’s 90% credible limits for GW170104 [ − 0.24:0.13]. Both BH masses are also within the limits: M_BH1 = 31.0 M_⊙ [25.4:38.2] and M_BH2 = 20.1 M_⊙ [15.6:25.0].