A seventh chord is a chord consisting of a triad plus a note forming an interval of a seventh above the chord's root. When not otherwise specified, a "seventh chord" usually means a dominant seventh chord: a major triad together with a minor seventh. However, a variety of sevenths may be added to a variety of triads, resulting in many different types of seventh chords, as described below.
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 passed and the collective ear 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 19th century reduced the dissonance of some earlier forms of sevenths.
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 (i.e. they are both major, minor, or diminished), the name is shortened. For instance, a major/major seventh is generally referred to as a major seventh. This rule is not valid for augmented chords: since the augmented/augmented chord is not commonly used, the abbreviation augmented is used for augmented/minor, rather than augmented/augmented. Additionally, half-diminished stands for diminished/minor, and dominant stands for major/minor. When the type is not specified at all, the triad is assumed to be major, and the seventh is understood as a minor seventh (e.g. a "C" chord is a "C major triad", and a "C7" chord is a "C major/minor seventh chord", also known as a "C dominant seventh chord").
The most common chords are tertian, constructed using a sequence of major thirds (spanning 4 semitones) and/or minor thirds (3 semitones). Seven tertian seventh chords are commonly found in western music:
Of all the seventh chords, perhaps the most important is the dominant seventh. It was the first seventh chord to appear regularly in classical music. The name comes from the fact that the flat seventh occurs naturally in the chord built upon the dominant (i.e. the fifth degree) of a given major diatonic scale. Take for example the C major scale (C, D, E, F, G, A, B, C):
The note G is the dominant degree of C major - its fifth note. 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 contains the four notes G-B-D-F and is called G dominant seventh (G7). The note F is a minor seventh from G, and is also called the dominant seventh with respect to G. However, the 'dominant' seventh is used on notes other than the dominant, such as the subdominant.
The harmonic seventh chord is a dominant seventh chord formed by a major triad plus an harmonic seventh interval.
The harmonic seventh interval is a minor seventh tuned in the 7:4 pitch ratio, one of the possible "just ratios" defined for this interval in just intonation (slightly below the width of a minor seventh as tuned in equal temperament). Sometimes called a "blue note", the harmonic seventh is 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.
Frequent use of the harmonic seventh 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. For example, C-E♭-G-B♭ can function as both a C minor seventh and an E flat added sixth (Id chord).
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 Δ7 (a delta chord) or just a Δ (which has the same meaning).
Half-diminished seventh chord
Leading-tone seventh chord (viiø7) in C major, bø7.
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 the root of a diminished triad.
Example: (in the key of C major) b-d-f-a.
Diminished seventh chord
A comparison of the Diminished 7th and Dominant 7th (b9) Chords
A diminished 7th chord is made of three superimposed minor 3rds (e.g. B-D-F-A♭), which is two tritones a minor third apart (e.g. B-F, D-A♭). 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. 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.
Inversions of seventh chord
There are four different inversions. Seventh chord inversions are as follows: