  Here we show explicitly the relationship between the functions used in the
atomic pair distribution function (PDF) method and those commonly used in small
angle scattering (SAS) analyses. The origin of the sloping baseline, $-4\pi
r\rho_0$, in PDFs of bulk materials is identified as originating from the SAS
intensity that is neglected in PDF measurements. The non-linear baseline in
nanoparticles has the same origin, and contains information about the shape and
size of the nanoparticles.
