A chord in music is any harmonic set of three or more notes that is usually heard simultaneously. These need not actually be played together: arpeggios and broken chords may for many practical and theoretical purposes be understood as chords. Chords and sequences of chords are frequently used in modern Western, West African and Oceanian music, whereas they are absent from the music of many other parts of the world. The use of chords is heavily influenced by the choice of instrument and the musician playing them.

The most frequently encountered chords are triads, so called because they consist of three distinct notes: further notes may be added to give seventh chords, extended chords, or added tone chords. The most common chords are the major and minor triads, and then the augmented and diminished triads. The I, IV and V chords are all major, whereas ii, iii and vi are minor.