In music, consonance and dissonance are categorizations of simultaneous or successive sounds. Consonance is associated with sweetness, pleasantness, and acceptability; dissonance is associated with harshness, unpleasantness, or unacceptability.
The terms form a structural dichotomy in which they define each other by mutual exclusion: a consonance is what is not dissonant, and reciprocally. However, a finer consideration shows that the distinction forms a gradation, from the most consonant to the most dissonant. As Hindemith stressed, “The two concepts have never been completely explained, and for a thousand years the definitions have varied” (Hindemith 1942, p. 85).
The opposition can be made in different contexts:
In acoustics or psychophysiology, the distinction may be objective. In modern times, it usually is based on the perception of harmonic partials of the sounds considered, to such an extent that the distinction really holds only in the case of harmonic sounds (i.e. sounds with harmonic partials).
In music, even if the opposition often is founded on the preceding, objective distinction, it more often is subjective, conventional, cultural, and style- and/or period-dependent. Dissonance can then be defined as a combination of sounds that does not belong to the style under consideration; in recent music, what is considered stylistically dissonant may even correspond to what is said to be consonant in the context of acoustics (e.g. a major triad in 20th century atonal music). A major second (e.g. the notes C and D played simultaneously) would be considered dissonant if it occurred in a J.S. Bach prelude from the 1700s; however, the same interval may sound consonant in the context of a Claude Debussy piece from the early 1900s or an atonal contemporary piece.In both cases, the distinction mainly concerns simultaneous sounds; if successive sounds are considered, their consonance or dissonance depends on the memorial retention of the first sound while the second sound (or pitch) is heard. For this reason, consonance and dissonance have been considered particularly in the case of Western polyphonic music, and the present article is concerned mainly with this case. Most historical definitions of consonance and dissonance since about the 16th century have stressed their pleasant/unpleasant, or agreeable/disagreeable character. This may be justifiable in a psychophysiological context, but much less in a musical context properly speaking: dissonances often play a decisive role in making music pleasant, even in a generally consonant context—which is one of the reasons why the musical definition of consonance/dissonance cannot match the psychophysiologic definition. In addition, the oppositions pleasant/unpleasant or agreeable/disagreeable evidence a confusion between the concepts of “dissonance” and of “noise”. (See also Noise in music, Noise music and Noise (acoustic).)
While consonance and dissonance exist only between sounds and therefore necessarily describe intervals (or chords), such as the perfect intervals, which are often viewed as consonant (e.g., the unison and octave), Occidental music theory often considers that, in a dissonant chord, one of the tones alone is in itself deemed to be the dissonance: it is this tone in particular that needs “resolution” through a specific voice leading procedure. For example, in the key of C Major, if F is produced as part of the dominant seventh chord (G7, which consists of the pitches G, B, D and F), it is deemed to be “dissonant” and it normally resolves to E during a cadence, with the G7 chord changing to a C Major chord.