Consonance
A pleasant-sounding sonority.
What it means
Consonance designates a sonority that rests in itself and needs no continuation. Its counterpart is dissonance, which demands resolution. Acoustically a simple frequency ratio underlies consonance: octave 2:1, fifth 3:2, fourth 4:3, major third 5:4, minor third 6:5. The simpler the ratio, the fewer beats arise and the calmer the sonority seems.
Musical use
The study of composition distinguishes perfect and imperfect consonances. Prime, octave, and fifth count as perfect — they sound hollow and merge strongly, which is why parallel motions in them are forbidden in strict writing: two voices running in fifths cease to be two voices. Third and sixth count as imperfect; they sound fuller, merge less, and may therefore be led in parallel. The fourth occupies a special position: above the bass it counts in strict writing as dissonant and in need of treatment, between upper voices as consonant.
Historically the classification is flexible. The early Middle Ages knew only octave, fifth, and fourth as full-fledged sonorities; the third long counted as imperfect and prevailed only gradually — its revaluation is connected with English influences of the fourteenth and fifteenth centuries and fundamentally changed the sound of European music.
Decisive is the interplay: consonance acts as resolution only because tension precedes it. Dissonance is therefore no deficiency but the condition for consonance to mean something.
The division into consonance and dissonance is not given by nature but culturally shaped and has changed. One distinguishes perfect consonances such as prime, octave, and fifth from imperfect ones such as thirds and sixths. Hermann von Helmholtz explained consonance in the nineteenth century through the absence of beats between the partials.
Performance practice
Intonate consonances by the beats — octaves and fifths first, then the thirds. Keep closing chords calm and without emphasis; they act through purity, not through force. And treat the dissonance before them as the actual event: whoever holds it back takes the effect from the resolution.