A detailed numerical investigation of the characteristics of the "limiting region" in the ionosphere is carried out, using both Booker's criterion (1936) and Budden's equation (1952) for determining the "limiting region". The results show that, in the case of vertically downcoming high-frequency radio waves, the "limiting region" always occurs at levels of very low electron density for all conditions of normal layer propagation, thus justifying the method of computing the limiting polarization (from the Appleton–Hartree equation) with N = 0.The possibility that irregular ionization patches in the lowermost ionosphere might give rise to multiple "limiting levels" and thus affect the limiting polarization of downcoming radio waves is investigated; it is shown that such an effect, though theoretically possible, is not likely to occur frequently.