Dissonance is a sonority in which the pitches “argue,” creating tension and a sense of incompleteness. Dissonances include the seconds, sevenths, the tritone, and all chords containing them. Classical theory required a dissonance to be prepared and always resolved into a consonance.
It is dissonance that gives music its motion: without tension seeking release, harmony would be static and dull. The history of music is the history of the gradual “emancipation of dissonance” — from the strict rules of the Baroque, through the bold sonorities of the Romantics, to the 20th century, when Schoenberg and others made dissonance an independent, equal means that no longer requires resolution. Dissonance is not a “mistake” but a source of expressiveness, drama, and the energy of musical development.
