The distance distribution function formula_13 is related to the frequency of certain distances formula_16 within the particle. Therefore it goes to zero at the largest diameter of the particle. It starts from zero at formula_17 due to the multiplication by formula_18. The shape of the formula_13-function already tells something about the shape of the particle. If the function is very symmetric, the particle is also highly symmetric, like a sphere. The distance distribution function should not be confused with the size distribution.