  The radial distribution function is a characteristic geometric quantity of a
point set in Euclidean space that reflects itself in the corresponding
diffraction spectrum and related objects of physical interest. The underlying
combinatorial and algebraic structure is well understood for crystals, but less
so for non-periodic arrangements such as mathematical quasicrystals or model
sets. In this note, we summarise several aspects of central versus averaged
shelling, illustrate the difference with explicit examples, and discuss the
obstacles that emerge with aperiodic order.
