It commonly agreed that the arrangement of atoms at the surface of a crystal is different from that of the bulk. Numerous simulation studies of extremely small crystallites have shown that in a few nanometers thick surface layer of a nanocrystal the interatomic distances are different from the "ordered bulk" values. As long as it's presence can be safely ignored for micrometer-sized crystallites such a layer constitutes a significant portion of a nanocrystal. The arrangement of atoms in a nanocrystal is not strictly periodic and therefore it must not described by a set of lattice parameter any more. Nevertheless diffraction patterns of the nanocrystalline samples show well developed diffraction peaks. Careful analysis of their positions shows that indeed, each diffraction peak of a nanometric material corresponds to a slightly different set of lattice parameters. This brings us to the concept of the apparent lattice parameter (alp), the quantity that does not describe the crystal as a whole but is hkl-dependent. In the presentation we will show that analysis of those small deviations of peak positions form the "expected" values can bring valuable information about the true atomic structure of nanocrystals. We will show results of numerical simulations of diffraction patterns of the nanocrystals with altered surface layers and compare them to the experimental data. We will also discuss the experimental techniques that must be used, methods of data evaluation and cases where the obtained information may become useful.