Groups of three or more notes can be classified as chords. Chords built from notes of a major or minor scale are known as diatonic (naturally belonging) to that key. Triads are three-note chords constructed by stacking thirds on a root note. The character of a triad is determined by two factors — the chord’s quality, determined by ratios between each note’s frequency; and the relationships between each note of the chord (most importantly its root note) with the tonic. The way a chord relates to the tonic, and by extension, the other chords within the key, is known as its function.
There are three qualities of chords which are diatonic to the major scale: major, minor, and diminished. Major triads consist of a major third, or two tones, between the bottom and middle notes, and a minor third, or a tone and a semitone, between the middle and top notes. The major chord appears three times across the repeating major scale pattern of tone-tone-semitone-tone-tone-tone-semitone: between degrees 1, 3, and 5; 4, 6, and 1; and 5, 7, and 2. The major chord is the most consonant chord, as it has the simplest ratios between its three notes: 4:5:6.
Minor triads consist of a minor third between the bottom and middle notes and a major third between the middle and top notes. The minor chord also appears three times within the major scale — between degrees 2, 4, and 6; 3, 5, and 7; and 6, 1, and 3. The minor chord is less consonant than the major chord, with a frequency ratio of 10:12:15, and is often described as having a “darker” sound.
The diminished chord consists of two stacked minor thirds. It appears once in the major scale - between degrees 7, 2, and 4. The diminished chord is the most dissonant triad diatonic to major keys, with a frequency ratio of 160:192:231.
In addition to their quality, each diatonic chord has one of three chord functions. The tonic chord is the tonal center of the key, where songs typically begin and end. Tonic chords typically feel at rest, without the inclination to resolve to another chord. Dominant chords possess a strong inclination to resolve to the tonic chord, based on the relationship between their notes and the tonic chord. Predominant chords possess a tendency to resolve to dominant chords. Predominant chords can be thought of as facilitating a departure from the tonic, while dominant chords enable a return. The tonic-predominant-dominant-tonic harmonic structure is fundamental to Western music.
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