Period ratio of a pair of adjacent planets versus the PR of the previous such pair for all 685 pairs in the period catalogue where every two adjacent pairs have PRs < 25. Hence, each data point represents two adjacent planet pairs, resulting in 413 points colour-coded based on the planet multiplicity in each system. There is a positive correlation, although the points are greatly dispersed around the 1:1 line. Points representing planet pairs in a system where there is a perfect spacing similarity lie precisely on top of each other on this line. The two black stars and diamonds mark the planet pairs in the inner and outer Solar System, respectively (Sect. 3.2).