Chords are the basis of harmony. The term chord refers to a group of notes (usually three or four) that are sounded and perceived as a single entity. A group of notes is usually heard as a chord when its notes are sounded simultaneously, rather than in succession, as part of a distinct melody. However, it is also possible for the notes of a chord to be performed in succession – i.e. as an arpeggio -- rather than simultaneously, without losing the sense that the combination of notes belong to a single harmony
A chord is described according to the number of notes in it, its fundamental note (its root), and the distances, or intervals,) between those notes (its quality). Chords containing three notes are called triads; chords containing four are typically seventh chords. Triads and seventh chords account for most of the chords in Western music, including both Classical and popular styles. The example below shows the four triads and five most commonly used seventh chords. Each chord in the example has the same root note, C.
Some musical traditions, particularly some Jazz styles, use chords with more than four notes. However, these chords (often called “extended harmonies”) are generally based on the chords shown here.
The number of potential linear combinations of the twelve pitches of the scale used in Western music that we hear as interesting or pleasing melodies is practically infinite. This is not true of vertical combinations of these pitches sounded simultaneously. Our ears perceive and appreciate relatively few chord combinations and sequences, and musicians must, therefore, continuously use these relatively few combinations and sequences. Because of this limitation, chords are considered part of the common stock of musical raw material, and are not copyrightable. Similarly, useful successions of chords—called “chord progressions”— because of their scarcity -- are not copyrightable.