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. Additionally, the general acceptance of equal temperament during the 1800s reduced the dissonance of some earlier forms of sevenths.
Of the eight possible constructions of seventh chords using major and minor thirds, seven are commonly found in western music (in addition to the synthetic "altered" seventh). They are built as indicated below:
In tuning systems other than equal temperament there are further possible seventh chords. In just intonation, for example, there are more sevenths than just major and minor. For example, the "harmonic seventh" (the 7:4 pitch ratio), sometimes called a "blue note", used by singers, through note bending on guitars, and on other instruments not restricted to equal temperament. An often heard example of the harmonic seventh chord is the last word of the modern addition to the song "Happy Birthday to You", with the lyrics, "and many more!" The harmony on the word "more" is typically sung as a harmonic seventh chord.
Of all the seventh chords, perhaps the most important to understand is the dominant seventh , a major triad with a minor seventh. It was the first seventh chord to appear regularly in Western music. The name comes from the fact that it occurs naturally in the seventh chord built on the dominant (fifth) scale degree of a given major key. Take for example the key of C major:
The note G is the dominant degree of C major, i.e., it is the fifth note of the major scale whose first note is C. When we arrange the notes of the C major scale in ascending pitch and use only these notes to build a seventh chord, and we start with G (not C), then the resulting chord, containing the four notes GBDF and called G7, is a major triad, GBD, with a dominant seventh, F.
This basic dominant seventh chord is useful to composers because it contains both a major triad and the interval of a tritone. The major triad confers a very "strong" sound. The tritone is created by the cooccurrence of the third degree and seventh degree (e.g., in the G7 chord, the acoustic distance between B and F is a tritone). 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 (e.g., from GBD to CEG in the key of C major), creates a resolution with which to end a piece or a section of a piece. Because of this original usage, it also quickly became an easy way to trick the listener's ear with a deceptive cadence.
However, the most important use of the dominant seventh chord in musical composition is the way that the introduction of a non-diatonic dominant seventh chord (sometimes called a chromatic seventh), 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.
The dominant seventh is frequently used to approximate a Harmonic Seventh (see below).
See also: irregular resolution
The harmonic seventh chord is a major triad plus the above-mentioned harmonic seventh interval. Frequent use of this chord is one of the defining characteristics of blues and barbershop harmony; barbershoppers refer to it as "the barbershop seventh." Since barbershop music tends to be sung in just intonation, the barbershop seventh chord may be accurately termed a harmonic seventh chord. The harmonic seventh chord is also widely used in "blues flavored" music. As guitars, pianos, and other equal-temperament instruments cannot play this chord, it is frequently approximated by a dominant seventh. As a result it is often called a dominant seventh chord and written with the same symbols (such as the blues progression I7 - V7 - IV7).
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-E♭-G-B♭ 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 dissonant, depending on the voicing used. For example, Bacharach and David's Raindrops Keep Fallin' on My Head opens with a major chord followed by a major seventh in the next measure.
The major seventh is sometimes notated as Δ 7 (a delta chord) or just a Δ (which has the same meaning).
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 enharmonically the same as a major sixth. The half-diminished seventh chord uses a minor seventh over a diminished triad.
The Diminished 7th Chord has been used by composers and musicians for a variety of reasons over time. Some reasons include: as a symbol of Sturm und Drang; modulation; and for characterisation. A diminished 7th chord is made of three superimposed minor 3rds (e.g. B-D-F-Ab) or two tritones a minor third apart (e.g. C-F#, Eb-A). The diminished 7th chord is seen more frequently in late classical and romantic period works but is also found in Baroque and Renaissance period works, though not as frequently.
All of the elements of the Diminished 7th chord can be found in the Dominant 7th (b9) chord as seen in a comparison of the two chords.