While harmonic distortion adds frequencies that are harmonically related, intermodulation distortion adds nonharmonic partials to the original signal. IM is a measure of how two frequencies that are present at the same time affect each other. Amplitude intermodulation distortion is caused when combinations of two or more frequencies generate new frequencies which are sums and differences of the original signal, i.e., the amplitude modulation of one signal by another. If an amplifier is used to amplify each tone equally, but if the gain of the amplifier varies with signal level (i.e., it is nonlinear), high-frequency sounds will be amplified by different amounts depending on whether a nearby low-frequency sound is near zero or near its peak. Therefore, the high-frequency signal will undergo changes in amplitude at the rate of the low-frequency signal, modulating the high-frequency sound which will be heard to flutter in the presence of the low-frequency sound. PIM is another component of intermodulation distortion, but to a lesser degree.