An effect created by two tones or signals played simultaneously, which are not at the exact same frequency, but are close enough so that the ear cannot distinguish them as two separate tones. The small difference in frequency causes the phase relationship between the two tones to change gradually, such that the two waveforms alternately reinforce and cancel each other. Depending on the waveforms involved, this produces a tremolo, a phase shifter effect, or some combination of the two. An extreme example of this can be demonstrated by combining two sine waves of identical amplitude but slightly different frequencies; at the moment where the two waves are 180 degrees out of phase, the result will be a moment of complete silence.
To a lesser extent, a beat frequency can also be heard between two tones when the frequency of one is close to, but not right on, an integer multiple of the frequency of the other. The equal-tempered tuning scale, which most Western musical instruments are based on, has many intervals between notes of the scale which fit this description. This is due to the compromise nature of equal-tempered tuning, which makes it practical to play in any musical key. Many alternate tunings, such as the "just intonation" tunings, try to address this problem by adjusting the frequency ratios between notes. The problem with such tunings is that they reduce beating between certain intervals at the cost of making them worse between other intervals, which necessarily makes them specific to a musical key.