Chords, Perception of

Common-Practice Chords

A triad is a specific type of common-practice chord, consisting of three tones stacked in thirds. The three members of the triad are referred to as the “root,” the “third,” and the “fifth”; the root is the lowest note in the stack of thirds, the third is the middle note (so called because it is a third above the root), and the fifth is the top note (so called because it is a fifth above the root).

Example 1: The triad. Common-practice triads are named based on the quality of the chord, determined by the intervals contained in the triad. A major triad consists of a major third above the root, with a minor third between the third and fifth; a minor triad consists of a minor third above the root, with a major third between the third and fifth. The interval between the root and fifth for both the major and minor triads is a perfect fifth. [Page 174]A diminished triad consists of two minor thirds stacked, resulting in a diminished fifth between the root and fifth; and an augmented triad consists of two major thirds stacked, resulting in an augmented fifth between the root and fifth.

Example 2: Types of common-practice triads. A “seventh chord” consists of four tones stacked in thirds, and gains its name because the interval between the root of the chord and the top note is a seventh. As before, seventh chords are conventionally named based on the quality and arrangement of intervals contained in the chord, with the name consisting of the quality of the triad first and then the quality of the seventh above the root; thus, for example, a major-minor seventh chord would contain a major triad and a minor seventh above the root, a half-diminished seventh chord consists of a diminished triad and a minor seventh above the root, and a fully diminished seventh chord contains a diminished triad with a diminished seventh above the root.

Example 3: Types of common-practice seventh chords. Other types of seventh chords are theoretically possible, but do not occur diatonically in Western common-practice tonal music.

If the root of a triad or a seventh chord is the lowest-sounding pitch in the chord, the chord is said to be in “root position.” If a note other than the root is the lowest-sounding pitch, the chord is described as being in “inversion.” Chords in inversion are less stable than root position chords.

Example 4: Triads and seventh chords in inversion. An example of a nondiatonic common-practice chord is the augmented sixth chord, so named for its characteristic interval of an augmented sixth between the lowered sixth scale degree of a key, typically in the lowest voice, and the raised fourth scale degree, typically in an upper voice. These notes tend to resolve outward to an octave on the dominant scale degree. There are three variations of augmented sixth chords, with regional but meaningless names to distinguish them.

...