A seventh chord is a chord consisting of a triad plus a note forming an interval of a seventh above the chord's root. In its earliest usage, the seventh was introduced solely as an embellishing or nonchord tone. The seventh destabilized the triad, and allowed the composer to emphasize movement in a given direction. As time progressed and the collective ears of the western world became more accustomed to dissonance, the seventh was allowed to become a part of the chord itself, and in some modern music, and jazz in particular, nearly every chord is a seventh chord.
Because a variety of sevenths may be added to a variety of chords, there are many types of seventh chords, depending on the type of triad and the quality of the seventh.
Types of seventh chords
Most textbooks name these chords formally by the type of triad and type of seventh; hence, a chord consisting of a major triad and a minor seventh above the root is referred to as a "major/minor seventh chord." When the triad type and seventh type are identical, the name is shortened; a major/major seventh is generally referred to as a "major seventh."
Of the eight possible constructions of seventh chords using major and minor thirds, five are most commonly found in western music. They are built as indicated below:
- Major Seventh (formally "major/major seventh", also maj7, M7, Δ7, ⑦): root, major third, perfect fifth, major seventh
- Minor Seventh (formally "minor/minor seventh", also m7): root, minor third, perfect fifth, minor seventh
- Major/Minor Seventh (commonly called dominant 7th even when not used in that context) (7): root, major third, perfect fifth, minor seventh
- Half Diminished (formally "diminished/minor seventh", a.k.a. "m7b5" or "minor seven[th,] flat five" among jazz musicians, also ø, m7b5): root, minor third, diminished fifth, minor seventh
- Diminished Seventh (formally "diminished/diminished seventh", also °7): root, minor third, diminished fifth (tritone), diminished seventh (sixth)
- Minor Major Seventh (also mM7, mΔ7, m⑦): root, minor third, perfect fifth, major seventh
- Altered Chord (7alt): root, flat second, sharp second, third, flat fifth, sharp fifth, flat seventh
The other two possible seventh chords – the augmented/major seventh and the augmented/minor seventh – are rarely seen in western music.
The dominant seventh
Of all the seventh chords, perhaps the most important to understand is the major triad with a minor seventh, also called the dominant seventh chord. Called the Dominant Seventh because its intervallic relationships occur naturally in the seventh chord built on the dominant scale degree of a given key, (eg. G7 in the key of C major) the dominant seventh chord was the first to begin to appear regularly in Western music.
The dominant seventh chord is useful to composers because it is a major chord with a very strong sound, that also includes a tritone between the third and seventh of the chord. In a diatonic context, the third of the chord is the leading-tone of the scale, which has a strong tendency to pull towards the tonal center, or root note, of the key. This, in combination with the strength of root movement by fifth, and the natural resolution of the dominant triad to the tonic triad, creates an incredibly satisfying resolution with which to end a piece. Because of this original usage, it also quickly became an easy way to trick the listener's ear with a deceptive cadence.
The most important usage, though, is the way that the introduction of a non-diatonic dominant seventh chord which is borrowed from another key, can allow the composer to modulate to that other key.
This technique is extremely common, particularly since the classical period, and has led to further innovative uses of the dominant seventh chord such as secondary dominant, extended dominant, and substitute dominant chords.
Frequent use of the dominant seventh is one of the defining characteristics of barbershop harmony; barbershoppers refer to it as "the barbershop seventh."
Major and Minor Seventh Chords
While the dominant seventh chord is typically built on the fifth (or dominant) degree of a major scale, the minor seventh chord is built on the second, third, or sixth degree. A minor seventh chord contains the same notes as an added sixth chord (see below under "Sixth chords") - for example, C-Eb-G-Bb can function as both a C minor seventh and an E flat added sixth.
Major seventh chords are usually constructed on the first or fourth degree of a scale, (in C or G major: C-E-G-B). Due to the major seventh interval between the root and seventh (C-B, an inverted minor second), this chord can sometimes sound a bit dissonant, depending on the voicing used. Example: Bacharach/David's "Rain Drops Keep Fallin' On My Head" opens with a major chord followed by a major seventh in the next measure.
Half-Diminished Seventh Chords
A half-diminished seventh chord is a seventh chord built from the seventh degree of a major scale. It's considered "half-diminished" because a fully diminished seventh has a double-flatted seventh, making it the same as a major sixth. The half-diminished seventh chord uses a minor seventh over a diminished triad. (Courtesy of Wikipedia)
