Joint posterior distributions of the parameters describing the binary progenitors of 3C 186. We show one- and two-dimensional marginal posteriors for the mass ratio (q), spin magnitudes (χ_1, 2), spin inclination angles (θ_1, 2), the in-plane azimuthal angle of the heavier BH spin (ϕ₁), the difference between the azimuthal angles (Δϕ), polar observation angles (θ_n), and the azimuthal observation angle (ϕ_n). Contours indicate 90% and 50% credible regions.