Abstract We report on the population of 47 compact binary mergers detected with a false-alarm rate of < 1 yr − 1 in the second LIGO–Virgo Gravitational-Wave Transient Catalog. We observe several characteristics of the merging binary black hole (BBH) population not discernible until now. First, the primary mass spectrum contains structure beyond a power law with a sharp high-mass cutoff; it is more consistent with a broken power law with a break at 39.7 − 9.1 + 20.3 M ⊙ or a power law with a Gaussian feature peaking at 33.1 − 5.6 + 4.0 M ⊙ (90% credible interval). While the primary mass distribution must extend to ∼ 65 M ⊙ or beyond, only 2.9 − 1.7 + 3.5 % of systems have primary masses greater than 45 M ⊙ . Second, we find that a fraction of BBH systems have component spins misaligned with the orbital angular momentum, giving rise to precession of the orbital plane. Moreover, 12 %– 44 % of BBH systems have spins tilted by more than 90°, giving rise to a negative effective inspiral spin parameter, χ eff . Under the assumption that such systems can only be formed by dynamical interactions, we infer that between 25% and 93% of BBHs with nonvanishing ∣ χ eff ∣ > 0.01 are dynamically assembled. Third, we estimate merger rates, finding R BBH = 23.9 − 8.6 + 14.3 Gpc

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