A chord which, when arranged in close position with its fundamental (or Root) in the bass, encompasses the interval of a 9th (ex.1a). The common functions of 9th chords and 11th and 13th chords reflect their construction and interpretation as upward extensions of triads and 7th chords. In the 20th century (especially in jazz and popular music) 9th chords are used as elaborations of simpler chords, particularly as substitutes for a tonic triad at the end of a piece; the ‘piling up’ of 3rds above the tonic to make a 7th, 9th, 11th or even 13th chord is one of the most important characteristics of jazz harmony. In tonal music up to about 1900, however, the 7th seems to have been the upper limit in chordal consonance; that is, while composers often used 9th and 11th chords for extra power, particularly at climaxes or final cadences, they invariably treated one or more notes in the chord as appoggiaturas (ex.1b). The thickness of a complete 9th, 11th or 13th chord in close position was also generally avoided, either by leaving out one or more of its notes (ex.1c) or by using a wider spacing (ex.1d).
For an 18th-century interpretation of 9th and 11th chords as downward extensions of 7th chords, see Supposition.
JULIAN RUSHTON