During the Common Practice Era, eight discreet sonorities comprised the primary harmonic resources for composers. Throughout the literature wherein a tertian vocabulary (chords built from stacking diatonic thirds) is employed, composers have sought to perpetuate the vitality of these simple harmonic structures through the invention of unique ways of handling their interrelationships, modifying the succession between one sonority and another, and cultivating the key associations from which they evolved and are inextricably tied.
1. Triads. The basic harmonic unit of tonal music is the diatonic triad, or the assembly of a succession of thirds from a given tonal scale. The root of the triad is the note on which the chord is based, while the third and fifth of the triad are the pitches built upon that root. Since there are only two qualities of intervals of thirds within the natural diatonic system, the major and minor third (labeled as M3 and m3 respectively), four possible triads can be constructed:
a) Major Triad - comprised of a M3 between the root and third, and a m3 between the third and fifth (analytical label: root as capital);
b) Minor Triad - comprised of a m3 between the root and third, and a M3 between the third and fifth (analytical label: root as lower case);
c) Diminished Triad - comprised of a m3 between the root and third, and a m3 between the third and fifth (analytical label: root as lower case followed by a small zero);
d) Augmented Triad - comprised of a M3 between the root and third, and a M3 between the third and fifth (analytical label: root as capital followed by a plus sign).
Note that the major and minor triads each contain a perfect fifth between the root and fifth of the chords. Their names are so given as each are the result of constructing triads from the tonic pitches of their respective modes, major and minor. The remaining two triads are labeled as such based on the relationship between the root and fifth of the triad; since a perfect fifth defines the strong harmonic resonance of the two fundamental triads (major and minor), the diminished triad contains a diminished fifth between the root and fifth, while the Augmented contains an augmented fifth between the same two segments.
2. Seventh Chords. A more complex sonority can be extrapolated from the diatonic triads by building an additional diatonic third above the fifth of the chord.
a) Dominant Seventh - comprised of a Major triad with a minor seventh between the root and seventh (analytical label: root as lower case followed by a 7);
b) Minor Seventh - comprised of a minor triad with a minor seventh between the root and seventh (analytical label: root as lower case followed by a 7);
c) Half-diminished Seventh - comprised of a minor triad with a minor seventh between the root and seventh (analytical label: root as lower case followed by a small slashed zero and a 7);
d) Diminished Seventh - comprised of a minor triad with a diminished seventh between the root and seventh (analytical label: root as lower case followed by a small zero and a 7).
Note, for the use of Arabic numerals to indicate chord positions, see the chapter on inversions.
