In a musical composition, a chord progression or harmonic progression is a succession of chords. Chord progressions are the foundation of harmony in Western musical tradition from the common practice era of Classical music to the 21st century. Chord progressions are the foundation of Western popular music styles (e.g., pop music, rock music) and traditional music (e.g., blues and jazz). In these genres, chord progressions are the defining feature on which melody and rhythm are built.
In tonal music, chord progressions have the function of establishing or contradicting a tonality, the technical name for what is commonly understood as the "key" of a song or piece. Chord progressions are usually expressed by Roman numerals in Classical music theory. For example, the common chord progression I–vi–ii–V. In many styles of popular and traditional music, chord progressions are expressed using the name and "quality" of the chords. For example, the previously mentioned chord progression, in the key of C major, would be written as C major–A minor–D minor–G major in a fake book or lead sheet. In the first chord, C major, the "C" indicates that the chord is built on the root note "C" and the word "major" indicates that a major chord is built on this "C" note.
In rock and blues, musicians also often refer to chord progressions using Roman numerals, as this facilitates transposing a song to a new key. For example, rock and blues musicians often think of the 12-bar blues as consisting of I, IV, and V chords. Thus, a simple version of the 12-bar blues might be expressed as I–I–I–I, IV–IV–I–I, V–IV–I–I. By thinking of this blues progression in Roman numerals, a backup band or rhythm section could be instructed by a bandleader to play the chord progression in any key. For example, if the bandleader asked the band to play this chord progression in the key of C major, the chords would be C–C–C–C, F–F–C–C, G–F–C–C; if the bandleader wanted the song in G major, the chords would be G–G–G–G, C–C–G–G, D–C–G–G; and so on.
The complexity of a chord progression varies from genre to genre and over different historical periods. Some pop and rock songs from the 1980s to the 2010s have fairly simple chord progressions. Funk emphasizes the groove and rhythm as the key element, so entire funk songs may be based on one chord. Some jazz-funk songs are based on a two-, three-, or four-chord vamp. Some punk and hardcore punk songs use only a few chords. On the other hand, bebop jazz songs may have 32-bar song forms with one or two chord changes every bar.