In a musical piece, a chord progression, also called a harmonic progression, is a sequence of chords played one after another. These progressions form the basis of harmony in Western music from the classical era to today. They are also central to many types of music, including pop, rock, traditional music, blues, and jazz. In these styles, chord progressions provide the structure for melodies and rhythms to develop.

In tonal music, chord progressions help establish or challenge the tonality, which is the technical term for the "key" of a song. For example, a common chord progression is written as I-V-vi-IV using Roman numerals in classical music theory. In popular and traditional music, chord progressions are often described by the names and qualities of the chords. For instance, in the key of E♭ major, the same progression would be written as E♭ major–B♭ major–C minor–A♭ major in a fake book or lead sheet. The term "E♭" shows the chord starts on the note E♭, and "major" means a major chord is used on that note.

In rock and blues, musicians often use Roman numerals to describe chord progressions because this makes it easier to change a song's key. For example, the 12-bar blues is commonly based on I, IV, and V chords. A simple version might be written as I–I–I–I, IV–IV–I–I, V–IV–I–I. Using Roman numerals allows a band to play this progression in any key. If a song is played in B♭ major, the chords would be B♭-B♭-B♭-B♭, E♭-E♭-B♭-B♭, F-E♭-B♭-B♭.

The complexity of chord progressions varies across music styles and time periods. Some pop and rock songs from the 1980s to 2010s use simple progressions. Funk music often focuses on rhythm and groove, so some songs may use only one chord. Jazz-funk songs might use two, three, or four chords in a repeating pattern. Punk and hardcore punk songs typically use a small number of chords. In contrast, bebop jazz songs may use 32-bar structures with one or two chord changes each bar.

Basic theory

A chord can be built on any note of a musical scale. This means a seven-note diatonic scale has seven basic triads, with each note acting as the root of a chord. A chord built on the note E is called an E chord, which can be major, minor, diminished, or another type. Some chords in a progression have more than three notes, such as seventh chords (like the V chord, which often resolves to the I chord) or extended chords. The role of a chord in a musical piece depends on its position within the chord progression.

When a major scale is harmonized, it creates three major triads. These triads are based on the first, fourth, and fifth notes of the scale. They are called the tonic chord (I), the subdominant chord (IV), and the dominant chord (V). These three chords can be used to harmonize all the notes in the scale. Many simple songs, such as folk music and rock and roll, often use only these three types of chords (for example, the song "Wild Thing" by The Troggs uses I, IV, and V chords).

The same major scale also includes three minor triads. These are the supertonic chord (ii), the mediant chord (iii), and the submediant chord (vi). These chords relate to each other in the same way as the major triads do in the relative minor key. For example, the relative minor of C major is A minor, and in A minor, the i, iv, and v chords are A minor, D minor, and E minor. In minor keys, the third note of the dominant chord is often raised by one semitone to form a major chord or a dominant seventh chord.

The seventh note of the major scale, called the leading tone, forms a diminished chord (vii).

A chord can also include chromatic notes, which are not part of the diatonic scale. One common chromatic change in simple folk music is raising the fourth note of the scale (♯) by one semitone. This often happens when the third note of the ii chord is raised, creating a chord that functions as the secondary dominant of the V chord (V/V). Chromatic notes are sometimes used to shift to a new key, which may later return to the original key, helping create a longer musical structure and a sense of movement.

While many chord progressions are possible, most are short, often lasting only a few bars. Some progressions are more commonly used than others, and certain patterns, like the 12-bar blues progression, help define musical styles.

In Western classical music, chords are labeled with Roman numerals. Other systems, such as figured bass and chord charts, are also used. These systems often allow or require some improvisation.

Common progressions

Diatonic scales, such as major and minor scales, are especially useful for creating common chords because they include many perfect fifths. These scales are often used in music where harmony is important, such as in Western classical music during the common practice period. In Arab and Indian music, where diatonic scales are also used, other non-diatonic scales are available. However, these musical traditions often avoid chord changes, staying on a single key-chord. This feature is also found in styles like hard rock, hip hop, funk, disco, and jazz.

Alternating between two chords is the simplest form of a chord progression. Many well-known musical pieces are built using just two chords from the same scale. For example, many classical melodies and popular songs, such as "Achy Breaky Heart," use the tonic (I) and dominant (V) chords. The Isley Brothers' song "Shout" uses the I–vi progression throughout.

Three-chord progressions are more common because they allow melodies to use any note in the scale. These progressions are often presented as four chords to create a binary harmonic rhythm, but two of the four chords are the same. Examples include:
• I – IV – V – V
• I – I – IV – V
• I – IV – I – V
• I – IV – V – IV
• I – IV – V – I

Sometimes, chords are chosen to match a melody, but often the progression itself inspires the melody.

Similar progressions appear in African popular music. These can be varied by adding sevenths or other scale degrees to chords or by replacing the IV chord with its relative minor, creating progressions like I–ii–V. This sequence is also used in jazz harmony, such as the ii–V–I turnaround.

Three-chord progressions form the harmonic base of much African and American popular music and appear in sections of classical music, such as the opening of Beethoven’s Pastoral Symphony.

When a simple sequence does not cover the entire harmonic structure of a piece, it can be extended for more variety. For example, a phrase with the progression I–IV–V–V, ending on an unresolved dominant, might be answered by a similar phrase that resolves back to the tonic chord, doubling the length of the structure.

Such passages may also alternate with different progressions to create simple binary or ternary forms, like the 32-bar structure.

The 12-bar blues and its variations use a longer version of the I–IV–V progression, which has produced many hit songs, including works by Chuck Berry and Little Richard. In its simplest form, the chord progression is:

Blues progressions have also been made more complex with chromatic elements, as seen in the "Bird blues."

Steedman (1984) suggested that a set of rules can generate all well-formed transformations in jazz, including basic blues changes and modified sequences like "rhythm changes." These transformations include:
• Replacing or adding a chord with its dominant, subdominant, or tritone substitution.
• Using chromatic passing chords.
• Applying the ii–V–I turnaround extensively.
• Altering chords, such as using minor chords or diminished sevenths.

Another way to expand the I–IV–V progression is by adding the chord of the sixth scale degree, creating sequences like I–vi–IV–V or I–vi–ii–V, sometimes called the 50s or doo-wop progression.

This progression was used in classical music and later became popular in songs like "Blue Moon" (1934) and "Heart and Soul" (1938). It was later used in pop music, such as in The Beatles’ "Happiness Is a Warm Gun."

Adding the ii chord to these progressions highlights their connection to circle progressions, named for the circle of fifths. These progressions use chords with roots in ascending fourth or descending fifth relationships, such as vi–ii–V–I. This type of progression is strong and common in music.

Classical composers often used short cyclical progressions by selecting chords from the series that complete a circle through all seven diatonic chords. These progressions were later used in American popular music, leading to variations like the ragtime and stomp progressions. Examples include the "rhythm changes" in George Gershwin’s "I Got Rhythm."

Harmonizing the scale

Music often uses repeating patterns of chords, but the ear also follows a step-by-step movement, where chords move up or down the scale. This is called a step progression because it follows the individual steps of the scale, making the scale itself act as a bassline. In the 17th century, composers favored descending bass lines for a technique called "divisions on the ground," which is seen in Pachelbel's canon, where similar harmonies follow the descending major scale.

A simple example of this descending pattern adds either the III or V chord to a common sequence like I–vi–IV–V. This addition allows the harmony to include the seventh note of the scale, creating a complete bassline like I–VII–VI.

In the final measures of the first movement of Ravel's Piano Concerto in G, a descending hybrid scale (phrygo-major) is harmonized. Ravel used a series of major triads in this pattern: G, F♯, E, D, C, B♭, A♭, G.

Minor and modal progressions

Similar methods used in major modes also work in minor modes. Songs can use one, two, or three minor chords, as seen in minor blues. An example of a descending minor chord pattern is the four-chord Andalusian cadence, which follows the sequence i–VII–VI–V.

Folk and blues music often uses the Mixolydian scale, which has a flattened seventh note. This changes the placement of major chords to I–VII–IV. For example, if the C major scale is used with G as the main note, the chords that originally appear on the first, fourth, and fifth degrees of C (C, F, and G) now appear on the first, fourth, and seventh degrees of G. A common chord pattern with these chords is I–♭VII–IV–I, which can also be played as I–I–♭VII–IV or ♭VII–IV–I–I.

In minor keys, moving up by a minor third to the related major key often leads to ascending scale progressions, especially those based on the ascending pentatonic scale. A typical pattern is i–III–IV (or iv)–VI.

According to Tom Sutcliffe:
This happened partly because the blues scale resembles modal scales and partly because of the guitar’s design. Using barre chords on the guitar allows the same chord shape to be moved up and down the neck without changing finger positions. This is also connected to the increased use of power chords in many rock music styles.