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.
