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. 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. For example, rock and blues musicians often think of the 12-bar blues as consisting of I, IV, and V chords.

In rock and blues, musicians also often refer to chord progressions using Roman numerals, as this facilitates transposing a song to a new key. In the first chord, E ♭ major, the 'E ♭' indicates that the chord is built on the root note 'E ♭' and the word 'major' indicates that a major chord is built on this 'E ♭' note. For example, the previously mentioned chord progression, in the key of E ♭ major, would be written as E ♭ major–B ♭ major–C minor–A ♭ major in a fake book or lead sheet. In many styles of popular and traditional music, chord progressions are expressed using the name and ' quality' of the chords.

Chord progressions, such as the extremely common chord progression I-V-vi-IV, are usually expressed by Roman numerals in Classical music theory. In tonal music, chord progressions have the function of either establishing or otherwise contradicting a tonality, the technical name for what is commonly understood as the ' key' of a song or piece. In these genres, chord progressions are the defining feature on which melody and rhythm are built. Chord progressions are the foundation of popular music styles (e.g., pop music, rock music), traditional music, as well as genres such as blues and jazz. Chord progressions are the foundation of harmony in Western musical tradition from the common practice era of Classical music to the 21st century. In a musical composition, a chord progression or harmonic progression (informally chord changes, used as a plural) is a succession of chords. The chords shown are F major, G major, and C major. A IV–V–I progression in the key of C major.

B flat blues chord progression