Editing Chord progression (section)

===Three-chord progressions===
{{See also|Three-chord song}}
Three-chord progressions 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.

* {{nowrap|I – IV – V – V}}
* {{nowrap|I – I – IV – V}}
* {{nowrap|I – IV – I – V}}
* {{nowrap|I – IV – V – IV}}

Often the chords may be selected to [[Harmonization|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 [[Seventh chord|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 [[Supertonic chord|ii chord]], is also used [[cadence (music)|cadentially]] in a common chord progression of [[jazz harmony]], the so-called [[ii–V–I progression|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|''Pastoral'' Symphony]]<ref>{{Cite web|url=https://guitaralliance.com/2013/01/chord-progressions/|title=Chord Progressions|website=guitaralliance.com|access-date=1 February 2019}}</ref>).

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 (music)|phrase]] has the progression I–IV–V–V, which ends on an [[Resolution (music)|unresolved]] [[Dominant (music)|dominant]], may be "[[Period (music)|answered]]" by a similar phrase that resolves back onto the [[tonic chord]], giving a structure of double the length:
:{| class="wikitable" style="text-align:center; width:110px;"

| width="25%" |I
| width="25%" |IV
| width="25%" |V
| width="25%" |V
|-
|I
|IV
|V
|I
|}

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