Consonance and Dissonance

A pair of terms used in discussions of harmony are consonance and dissonance, meaning (roughly speaking) chords that sound at rest and those that sound tense, respectively. Discord is another term for dissonance. These qualities depend on the particular combinations of pitches that are sounding simultaneously to make up these chords. Octaves are the most consonant of intervals. Half steps are the most dissonant, as you can hear by striking any two adjacent keys on a piano at the same time.

“Medicine, to produce health, must know disease; music, to produce harmony, must know discord.”

Plutarch, c. 46–120C.E.

In everyday language, discord implies something unpleasant; discordant human relationships are to be avoided. But music does not avoid dissonance, for a little discord supplies the subtle tensions that are essential to make music flow along. A dissonant chord leaves a feeling of expectation; it seems to demand a consonant chord following it to complete the gesture and to make the music come to a point of stability. This is called resolution; the dissonance is said to be resolved. Without dissonance, music would be bland, like food without salt or spices.