A chord, in music, is any harmonic set of pitches consisting of two or more (usually three) notes (also called "pitches") that are heard as if sounding simultaneously. In everyday use by musical ensembles such as bands and orchestras, the three or more notes of a chord are often sounded together. However, the notes of a chord do not have to be played together at the same time: arpeggios and broken chords (these involve the notes of the chord played one after the other, rather than at the same time) may, for many practical and theoretical purposes, also constitute chords. Chords and sequences of chords are frequently used in modern West African and Oceanic music, Western classical music, and Western popular music; yet, they are absent from the music of many other parts of the world. In tonal Western classical music (music with a tonic key or "home key"), the most frequently encountered chords are triads, so called because they consist of three distinct notes: the root note, and Intervals of a third and a fifth above the root note. Further notes may be added to give tetrads such as seventh chords, the most commonly encountered example being the dominant seventh chord, consisting of the root note, the major third, the perfect fifth, and the minor seventh above the root.
Other chords with more than three notes include added tone chords, extended chords and tone clusters, which are used in contemporary classical music, jazz and other genres. Triads commonly found in the Western classical tradition are major and minor chords, with augmented and diminished chords appearing less often. The descriptions major, minor, augmented, and diminished are referred to collectively as chordal quality. Chords are also commonly classified by their root note—for instance, a C major triad consists of the pitch classes C, E, and G. A chord retains its identity if the notes are stacked in a different way vertically; however, if a chord has a note other than the root note as the lowest note, the chord is said to be in an inversion of the original chord (this is also called an "inverted chord"). While most chords have at least three notes, power chords, which are widely used in rock music, particularly in hard rock and heavy metal music, have just two notes: the root and the fifth (although the root may be doubled with the octave above).
An ordered series of chords is called a chord progression. One example of a widely used chord progression in Western traditional music and blues is the 12 bar blues progression, the simplest versions of which include tonic, subdominant and dominant chords (this system of naming chords is described later in this section). Although any chord may in principle be followed by any other chord, certain patterns of chords are more common in Western music, and some patterns have been accepted as establishing the key (tonic note) in common-practice harmony–notably the movement between tonic and dominant chords. To describe this, Western music theory has developed the practice of numbering chords using Roman numerals which represent the number of diatonic steps up from the tonic note of the scale.