Chord progression

The diatonic harmonization of any major scale results in three major triads, which are based on the first, fourth, and fifth scale degrees. The triads are referred to as the tonic chord (in Roman numeral analysis, symbolized by "I"), the subdominant chord (IV), and the dominant chord, (V), respectively.[3] These three triads include, and therefore can harmonize, every note of that scale. Many simple traditional music, folk music and rock and roll songs use only these three chord types (e.g. The Troggs' "Wild Thing", which uses I, IV and V chords).

The same major scale also has three minor chords, the supertonic chord (ii), mediant chord (iii), and submediant chord (vi), respectively. These chords stand in the same relationship to one another (in the relative minor key) as do the three major chords, so that they may be viewed as the first (i), fourth (iv) and fifth (v) degrees of the relative minor key. For example, the relative minor of C major is A minor, and in the key of A minor, the i, iv and v chords are A minor, D minor and E minor. In practice, in a minor key, the third of the dominant chord is often raised by one semitone to form a major chord (or a dominant seventh chord if the seventh is added).

In addition, the seventh degree of the scale (i.e. the leading tone) forms a diminished chord (vii^o).[4]

A chord may also have chromatic notes, that is, notes outside of the diatonic scale. Perhaps the most basic chromatic alteration in simple folk songs is the raised fourth degree (♯) that results when the third of the ii chord is raised one semitone. Such a chord typically functions as the secondary dominant of the V chord (V/V). In some instances, chromatic notes are introduced to modulate to a new key. This in turn may lead to a resolution back to the original key later on, so that the entire sequence of chords helps create an extended musical form and a sense of movement.

Although there are many possible progressions, in practice, progressions are often limited to a few bars' lengths and certain progressions are favored above others. There is also a certain amount of fashion in which a chord progression is defined (e.g., the 12-bar blues progression) and may even help in defining an entire genre.[5]

In western classical notation, chords are numbered with Roman numerals. Other types of chord notation have been devised, from figured bass to the chord chart. These usually allow or even require a certain amount of improvisation.

Diatonic scales such as the major and minor scales lend themselves particularly well to the construction of common chords because they contain many perfect fifths. Such scales predominate in those regions where harmony is an essential part of music, as, for example, in the common practice period of western classical music. In considering Arab and Indian music, where diatonic scales are used, there are also available a number of non-diatonic scales, the music has no chord changes, remaining always upon the key-chord, an attribute which has also been observed in hard rock, hip hop,[6] funk, disco,[7] jazz, etc.

Alternation between two chords may be thought of as the most basic chord progression. Many well-known pieces are built harmonically upon the mere repetition of two chords of the same scale.[2] For example, many of the more straightforward melodies in classical music consist entirely or mostly of alternation between the tonic (I) and the dominant (V, sometimes with an added seventh), as do popular songs such as "Achy Breaky Heart".[8] The Isley Brothers' "Shout" uses I–vi throughout.[9]

Three-chord progression are more common since a melody may then dwell on any note of the scale. They are often presented as successions of four chords (as shown below), in order to produce a binary harmonic rhythm, but then two of the four chords are the same.

• I–IV–V–V
• I–I–IV–V
• I–IV–I–V
• I–IV–V–IV

Often the chords may be selected to fit a pre-conceived melody, but just as often it is the progression itself that gives rise to the melody.

Similar progressions abound in African popular music. They may be varied by the addition of sevenths (or other scale degrees) to any chord or by substitution of the relative minor of the IV chord to give, for example, I–ii–V. This sequence, using the ii chord, is also used cadentially in a common chord progression of jazz harmony, the so-called ii–V–I turnaround.

Three-chord progressions provide the harmonic foundation of much African and American popular music, and they occur sectionally in many pieces of classical music (such as the opening bars of Beethoven's Pastoral Symphony[10]).

Where such a simple sequence does not represent the entire harmonic structure of a piece, it may readily be extended for greater variety. Frequently, an opening phrase has the progression I–IV–V–V, which ends on an unresolved dominant, may be "answered" by a similar phrase that resolves back onto the tonic chord, giving a structure of double the length:

I	IV	V	V
I	IV	V	I

Additionally, such a passage may be alternated with a different progression to give a simple binary or ternary form such as that of the popular 32-bar form (see musical form).

Blues progressions influenced a great deal of 20th century American popular music

The 12-bar blues and its many variants use an elongated, three-line form of the I–IV–V progression that has also generated countless hit records, including the most significant output of rock and rollers such as Chuck Berry and Little Richard. In its most elementary form (and there are many variants), the chord progression is

I	I	I	I
IV	IV	I	I
V	IV	I	I

Blues progressions have also been subjected to densely chromatic elaboration, as in the Bird blues.

Steedman (1984) proposed that a set of recursive rewrite rules generate all well-formed transformations of jazz, both basic blues chord changes and slightly modified sequences (such as the "rhythm changes"). Important transformations include:

• replacement of (or addition to) a chord with its dominant, subdominant or the tritone substitution.
• use of chromatic passing chords.
• extensively applying the ii–V–I turnaround.
• chord alterations such as minor chords, diminished sevenths, etc.[11]

A 50s progression in C

Another common way of extending the I–IV–V progression is by adding the chord of the sixth scale degree, giving the sequence I–vi–IV–V or I–vi–ii–V, sometimes called the 50s progression or doo-wop progression.

This progression had been in use from the earliest days of classical music and then generated popular hits such as Rodgers and Hart's "Blue Moon" (1934)[12] and Hoagy Carmichael's "Heart and Soul" (1938).[13]

Taken up into the pop mainstream, it continued to be used sectionally, as in the last part of The Beatles' "Happiness Is a Warm Gun".[14]

A circle progression in C major

The ragtime progression (E⁷–A⁷–D⁷–G⁷) often appears in the bridge of jazz standards.[15] The III⁷–VI⁷–II⁷–V⁷ (or V⁷/V/V/V–V⁷/V/V–V⁷/V–V⁷) leads back to C major (I) but is itself indefinite in key.

Introducing the ii chord into these progressions emphasises their appeal as constituting elementary forms of circle progression. These, named for the circle of fifths, consist of "adjacent roots in ascending fourth or descending fifth relationship"—for instance, the sequence vi–ii–V–I ascends with each successive chord to one a fourth above the previous. Such a motion, based upon close harmonic relations, offers "undoubtedly the most common and the strongest of all harmonic progressions".[16] Short cyclical progressions may be derived by selecting a sequence of chords from the series completing a circle from the tonic through all seven diatonic chords:[16]

I–IV–vii^o–iii–vi–ii–V–I

This type of progression was much used by classical composers, who introduced increasingly subtle inflections. Particularly, substitution of major for minor chords giving, for example, I–VI–II–V allowed a more sophisticated chromaticism as well as the possibility of modulation. These harmonic conventions were taken up by American popular entertainers, giving rise to many variations on those harmonic staples of early jazz that have been dubbed the ragtime progression and the stomp progression. All such progressions may be found used sectionally, as for example in the much-used "rhythm changes" of George Gershwin's "I Got Rhythm".