Atomic ordering of strongly nonstoichiometric compounds like carbides of transition metals is a very widespread phenomenon. The lower tungsten carbide \(\mathrm{W}_{2}\mathrm{C}_{y}\) is a strongly nonstoichiometric interstitial compound and has a wide homogeneity interval \(\mathrm{WC}_{0.34}-\mathrm{WC}_{0.52}\) at a temperature \(\sim \)3000 K. The literature data on the crystal structure of disordered and different ordered phases of \(\mathrm{W}_{2}\mathrm{C}_{y}\) carbide are contradictory. In this chapter the symmetry analysis of all possible superstructures of \(\mathrm{W}_{2}\mathrm{C}_{y}\) is performed and the physically possible sequence of phase transformations in this carbide is established. Also an analytical method is proposed for calculating the probabilities of existence of different pair interactions in the nonmetal sublattice of superstructures formed in strongly nonstoichiometric compounds \(\mathrm{MX}_{y}\) and \(\mathrm{M}_{2} \mathrm{X}_{y^{\prime }}\) with a high content of structural vacancies \(\square \).