The generalised thin-film optical method is used to analyze propagation phenomena in a typical night-time ionosphere at frequencies in the range 10–300 kHz, the ‘critical’ frequency being 24 kHz. Propagation phenomena in the VLF and lower LF range take place in conditions of ‘limiting polarisation’, so that the ‘composite’ or physical waves pass quite smoothly through the X = 1 level and other regions of rapidly varying polarization, even at the critical frequency. In such cases the inter-mode coupling coefficients attain very large values which are just sufficient to produce the requisite ‘mode reshuffling’ to maintain composite waves of almost constant polarisations and amplitudes. The low values of electron density in the D-layer lead to large departures from quasilongitudinal (Q.L.) propagation conditions, which are generally reestablished however in the region of large and rapid increase of electron density at the top of the D-layer. This leads to ‘mode reshuffling’, and if the X = 1 level is at a somewhat greater altitude the coupled ordinary wave may then be reflected at the X = 1 level to produce effects analogous to the Z-coupling echo and the Z-trace. In the upper LF range the inter-mode coupling processes associated with the rapid variation of wave polarisation at the X = 1 level are suppressed by an interference process so that ‘mode reshuffling’ is eliminated, and an initial extraordinary wave follows the characteristic polarisation reversal at the X = 1 level with little loss.