Distribution of first-order MMRs among exoplanet systems in 3+ planet resonant chains as a function of their orbital period. The colour indicates the first-order MMR in which resonant planet pairs are. See Sect. 6 for the selection process. These resonant chains are consistent with a formation by type I migration. As discussed in Sect. 4, low-index (2:1 and 3:2) MMRs tend to belong in uniform resonant chains, while at higher indexes, the inner pair is usually in a higher-index MMR with respect to the outer pair.