Compounds (299) containing 494 symmetrically independent pyridine-2,6-dicarboxylate moieties have been investigated. Among them the structures of Na(3)[Nd(Pydc)(3)].14H(2)O and Na(3)[Er(Pydc)(3)].11.5H(2)O, where H(2)Pydc is pyridine-2,6-dicarboxylic acid, were determined by single-crystal X-ray diffraction, while the others were taken from the Cambridge Structural Database. The characteristics of any complex by means of the ;method of crystallochemical analysis' are described, and the coordination types of all the Pydc ions and crystallochemical formulae of all the compounds were determined. Although the ion can act as a mono-, bi-, tri-, tetra- and pentadentate ligand, 96% of Pydc ions are coordinated to the central A atom in the tridentate-chelating mode. The dependence of the denticity and geometry of pyridine-2,6-dicarboxylate, as well as of the composition of Pydc-containing complexes, was studied as a function of the nature of the A atom, the molar ratio Pydc:A and the presence of neutral or acidic ligands in the reaction mixture.