Histograms of the period ratios of planets in compact systems. We distinguish between compact systems that are stable and likely to become unstable within the next 10–100 Myr following the procedure outlined in Section 2.2.2 (red and blue histograms, respectively). In the stable systems, we find that 30% of the planetary pairs are within 0.1 P_j+1/P_j of mean-motion resonances (7%, 15%, 4%, and 5% have period ratios 3:2, 2:1, 3:1, and 4:1 respectively). In the systems that we predict to be unstable, we find that 50% of the planetary pairs are within 0.1 P_j+1/P_j of mean-motion resonances (28%, 16%, 3%, and 2% have period ratios 3:2, 2:1, 3:1, and 4:1 respectively). Overall, this translates in 37% of the planetary pairs in the compact systems being within 0.1 P_j+1/P_j of the 3:2, 2:1, 3:1, and 4:1 mean-motion resonances. System-wise, 20% of the stable systems have planets in near–mean-motion resonances, while 70% of the unstable systems have planets in near–mean-motion resonances.