Dissonance
A sonority that demands resolution; the frequency ratios are complex.
What it means
A dissonance is a sonority that does not rest in itself but demands continuation. Acoustically it is based on complex frequency ratios: the partials of the notes involved do not coincide, beats arise, and the sound seems tense. Seconds, sevenths, the tritone, and all augmented and diminished intervals count as dissonant; the fourth above the bass occupies a special position and is likewise treated as dissonant in strict writing.
Musical use
What is decisive is that dissonance is not a deficiency but the actual means of musical tension. Without it there would be no resolution, and a consistently consonant music seems powerless. The study of composition regulates its use in three steps: preparation — the dissonant note first sounds as a consonance in the same voice; dissonance — it stays put while the other voices move on; resolution — it passes stepwise downward into a chord note. The suspension follows exactly this pattern.
Historically the treatment has loosened steadily. In the vocal polyphony of the Renaissance every dissonance is prepared and regulated; in the Baroque the dominant seventh chord (V7) is added as a sonority that can be used freely; in the nineteenth century resolutions are deferred or reinterpreted, and in the twentieth century the distinction loses its binding force in wide areas.
Dissonance is to be distinguished from cacophony: the former is a technical term, the latter a judgment.
Which intervals count as dissonant has changed in the course of history. In the Middle Ages thirds counted as imperfect, later as consonances; the fourth changed its status depending on the voice register. In the twentieth century dissonance was finally freed from the obligation to resolve.
Performance practice
Do not take dissonances back — they want to be audible, otherwise the resolution loses its effect. Give the dissonant note weight and the resolution relaxation, not the other way round. With flexible intonation, take care not to smooth away the tension by adjusting. And seek out the dissonances deliberately when working up a piece; they are usually the most revealing places of a movement.