The simultaneous sounding of two or more tones which, because of beats,
seems unpleasant or tense to the human ear; the converse of consonance.
Mathematical definitions of consonance, and by association dissonance, go
back to Pythagoras in the 5th century BC. Which
musical intervals are considered dissonant
is largely a matter of cultural and historical relativism. For instance,
some intervals regarded as dissonant in the Medieval period were found to
be consonant during the Renaissance. Also, the way a dissonance is resolved
(and even the way it is orchestrated) can decrease or intensify how harsh
it sounds. Intervals commonly considered dissonant in tonal music are the
major and minor seconds and sevenths. Consonance and dissonance are in relation
and, as such, are relative terms.
A dissonant chord, or dischord, is a chord
that sounds incomplete until it resolves itself on a harmonious chord.
See also suspension.