Distribution of the orbital PRs of pairs of adjacent planets in the period catalogue up to a PR limit of 25 shown for each planet multiplicity as a stacked histogram: systems with three (magenta), four (green), five (blue), and six to eight (red) detected planets. This sample contains 695 planet pairs, thereby excluding 14 pairs with 25 < PRs < 2662. Adjacent planets in higher-multiplicity systems tend to have smaller PRs. There are overabundances of PRs slightly greater than the 4:3, 3:2, and 2:1 MMRs (Sect. 3.1).