The kDB system, consisting of the propagation vector k and the plane perpendicular to k and containing the field vectors D and B, is developed to treat electromagnetic waves in anisotropic and bianisotropic media. In using the kDB system, we first transform from the original coordinate systems to the kDB, determine the characteristics of the normal modes, and then transform the results back to the original coordinates.
J. A. Kong, "Optics of bianisotropic media," J. Opt. Soc. Am. 64, 1304-1308 (1974)