The salient features of the sidebands generated on the HF waves in an ionospheric plasma modified by two pump frequencies are presented and compared to theories. Particular attention is focused upon the properties of the nonlinear modulation process by comparing the modulation index theory and the observations. The observations show that the amplitudes of the sidebands are related to the HF power and the difference frequency of the two pump waves. The relationships of the amplitude of the sideband with HF frequency, the scale height, and the temperature of the electrons in the ionosphere are discussed. The modulation index theory shows agreement with the observations over a range of conditions.