Linear coupling of electromagnetic waves in weakly inhomogeneous non-one-dimensional media is considered as a manifestation of the polarization degeneracy of the Maxwell equations. It is shown that the presence of two polarization-degenerate normal waves imposes strong constraints on the dielectric tensor components near the interaction region. As a result, the possible types of linear wave coupling and the corresponding wave equations admit a universal classification, which is independent of the way in which the linear medium is modeled.