In an Arithmetic Shellworld, the levels were regularly spaced at fourteen-hundred-kilometre intervals. Exponential or Incremental Shellworlds had more levels close to the core and fewer further out as the distance between each successive shell increased according to one of a handful of logarithm-based ratios. Arithmetic Shellworlds invariably held fifteen interior surfaces and were forty-five thousand kilometres in external diameter. Incremental Shellworlds, forming about twelve per cent of the surviving population, varied. The largest class was nearly eighty thousand kilometres across.
