Another special case of light propagation in an anisotropic crystal is that of propagation along a principal plane, such as the XY, YZ or ZX planes. We start with the propagation vector k lying in the YZ plane, so that mX = 0 and . With mX = 0, Eqns. 1.55 give
The two cases of nX < nY < nZ and nX > nY > nZ are considered separately in the next two subsections.