The second basic theorem of lattice crystallography (by N. V. Belov) underlies the theory of atomic sublattices characteristic of the overwhelming majority of crystal structures we have studied. By the example of structures of particular inorganic compounds (involving mercury chalcohalides) it is illustrated that a priory knowledge of cationic and anionic sublattices typical of a given class of compounds may provide useful information for analyzing powder diffraction patterns of substances with unknown crystal structures. It has been shown that complicated structures can be analyzed “in parts” by identifying their individual components discernable in powder diffraction patterns.