Size dispersion σ_R versus spacing dispersion σ_PR (Eq. (2)) of all pairs of adjacent planets in a system. This shows the 206 systems from the radius catalogue in which all adjacent planet pairs have PRs < 6. There is no correlation between the two dispersions. Among the points enclosed by the red lines, there are 51 systems in which all PRs < 2. The black squares represent the inner regions of five systems that contain a minimum of three inner small planets and at least one outer giant. Both the inner Solar System (square) with the terrestrial planets and the outer Solar System (triangle) with the four giants possess typical dispersions (Sect. 4).