In music, extended chords are certain chords (built from thirds) or triads with notes extended, or added, beyond the seventh. Ninth, eleventh, and thirteenth chords are extended chords. The thirteenth is the farthest extension diatonically possible as, by that point, all seven tonal degrees are represented within the chord (the next extension, the fifteenth, is the same as the root of the chord). In practice however, extended chords do not typically use all the chord members; when it is not altered, the fifth is often omitted, as are notes between the seventh and the highest note (i.e., the ninth is often omitted in an eleventh chord; the ninth and eleventh are usually omitted in a thirteenth chord), unless they are altered to give a special texture.
Chords extended beyond the seventh are rarely seen in the Baroque era, and are used more frequently in the Classical era. The Romantic era saw greatly increased use of extended harmony. Extended harmony prior to the 20th century usually has dominant function – as V, V, and V, or V/V, V/ii etc.
