Polar optical phonons in the binary AlN/GaN superlattices (SL) were studied in the framework of dielectric continuum model (DCM). It is shown that the modes propagating in the interface plane are delocalized over all layers and the modes propagating along the SL axis are delocalized in certain layers. Frequencies of former strongly depend on the layer thickness ratio and frequencies of the latter do not depend on the SL structure. Such behavior is typical for the short-period SL. We show that a strong coupling between two types of the modes takes place at increasing SL period. The phenomenon is described mathematically by the frequency- structure relations and its physical meaning is discussed.