Editing Chord progression (section)

==Basic theory==
[[File:Chord progression.png|thumb|338px|The key note, or [[Tonic (music)|tonic]], of a piece of music is called note number one, the first step of (here), the ascending scale iii–IV–V. Chords built on several [[scale degree]]s are numbered likewise. Thus the chord progression E minor–F–G can be described as three–four–five, (or iii–IV–V).]]

A chord may be built upon any note of a [[musical scale]]. Therefore, a seven-note [[diatonic scale]] allows seven basic [[Diatonic chord|diatonic triads]], each degree of the scale becoming the [[Root (music)|root]] of its own chord.<ref>[[George Whitefield Chadwick]], ''Harmony: A Course of Study'', 72nd ed. B. F.Wood Music, 1922, Introduction p. xv</ref> A chord built upon the note E is an E chord of some [[Chord quality|type]] (major, minor, diminished, etc.) Chords in a progression may also have more than three notes, such as in the case of a [[seventh chord]] (V<sup>7</sup> is particularly common, as it resolves to I) or an [[extended chord]]. The harmonic [[Function (music)|function]] of any particular chord depends on the context of the particular chord progression in which it is found.<ref name="Schoenberg">[[Arnold Schoenberg|Schoenberg, Arnold]]. ''Structural Functions of Harmony'', Norton, 1954, p. 1.</ref>

=== Diatonic and chromatic chords ===
The diatonic harmonization of any [[major scale]] results in three [[Major chord|major triads]], which are based on the first, fourth, and fifth [[scale degree]]s. 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.<ref>Chadwick, 1922, p.1</ref> These three triads include, and therefore can [[harmonization|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 (The Troggs song)|Wild Thing]]", which uses I, IV and V chords).

The same major scale also has three [[minor chord]]s, the [[Supertonic|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 (chord)|third]] of the dominant chord is often raised by one semitone to form a major chord (or a [[dominant seventh chord]] if the [[Seventh (chord)|seventh]] is added).

In addition, the seventh degree of the major scale (i.e. the [[Leading-tone|leading tone]]) forms a [[Diminished triad|diminished chord]] (vii{{music|dim}}).<ref>C. H. Kitson, ''Elementary Harmony, Part One'', Oxford University Press, 1920, chapters 3–4.</ref>

A chord may also have [[Diatonic and chromatic|chromatic]] notes, that is, notes outside of the diatonic scale. Perhaps the most basic chromatic [[Altered chord|alteration]] in simple folk songs is the raised fourth degree ({{music|sharp}}{{music|scale|4}}) that results when the [[Third (chord)|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 [[Modulation (music)|modulate]] to a new key. This in turn may lead to a [[Resolution (music)|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.

=== Progressions ===
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 [[Twelve-bar blues|12-bar blues]] progression) and may even help in defining an entire [[Music genre|genre]].{{citation needed | date=June 2017}}

In western [[Classical music|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 (music)|improvisation]].