Classes of media can also be defined by requiring that plane waves in such media are restricted by certain conditions. The chapter considers only two examples: first, media in which the wave vector k or the corresponding wave one‐form V of a plane wave can be freely chosen without being restricted by a dispersion equation and, second, media in which plane waves are required to be decomposable in two sets in terms of their polarization properties. The media thus defined are respectively called “media with no dispersion equation” (NDE media) and “decomposable media“ (DC media). Various subclasses of NDE media and DC media are presented in this chapter. The quartic dispersion equation can be split in two quadratic equations corresponding to the decomposed waves. The converse, starting from the splitting of the quartic equation and deriving the polarization decomposition, can also be done.