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In music, dissonance is the quality of sounds which seem "unstable", and have an aural "need" to resolve to a "stable" consonance. Both are words applied to harmony, chordss, and intervalss. The most strict definition of dissonance includes all sounds which are considered "unpleasant", while the most general definition includes only those which are restricted in their use.

Dissonance has been defined differently by different people in different places at different times. Western musical history can be seen as progessing towards a wider definition of consonance, culminating in the "emancipation of the dissonance", such as in the view of Arnold Schoenberg. Henry Cowell viewed tone clusters as the use of higher and higher overtones. While the perception and definition of dissonance is obviously culturally influenced, dissonance may also have some objective basis. In general the closer the frequencies of two pitches the more dissonant they are. As two pitches approach each other they begin to produce beat oscillations, which are caused when the two pitches cause interference, or reinforce and cancel each others amplitudes. Consonance between two notes may also be defined as greater coincidence of their harmonics or partials, which collectively are overtones. Dissonance is then defined by the amount of beating between non-common harmonics. By this definition dissonance is dependent not only on the quality of the interval between two notes, but the harmonics and thus sound quality (timbre) of those notes themselves.

There is a distinction between melodic and harmonic dissonance. For instance, a minor second is highly dissonant harmonically but not melodically, while the tritone is often considered a harmonic and melodic dissonance.

In what is now called the common practice period dissonant melodic intervals include the tritone and all augmented and diminished intervals. Dissonant harmonic intervals include:

This is as would be taught in a beginning theory class, but intervals such as the perfect fourth and thirds were once considered forbidden dissonances.

See also Cognitive dissonance, consonance.

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In poetry, dissonance is the deliberate avoidance of patterns of repeated vowel sounds (see assonance).