Using values of the frequencies vc for the high-frequency cutoffs of 31 pulsars obtained by other authors, we confirmed the relationship (obtained by us earlier) between vc and the period of pulsar P. The plot vc(χ) was drawn where χ is the relative angular distance between the line of sight and the center of the radiation cone. It is shown that pulsars with long periods cluster in the lower left corner of this diagram. Some geometrical and physical reasons causing high-frequency cutoffs are analyzed. The results can be explained by a model of thin plasma sheets. The interaction of these sheets causes generation of Langmuir waves at distances ~108 cm from the center of the neutron star. Pulsars without high-frequency cutoffs can be described by a model in which vc is very near the frequency of maximum in the spectra of these pulsars.