The ninth chord in music results from stacking an additional third atop the seventh degree, and so is generally considered to be an extension of the seventh chord. A few common types of ninth chord are shown here, in closed voicing:
Ninth chords of various kinds are especially prominent in jazz and jazz-inflected music, where they function as analogues or substitutions for their related sevenths as a way of adding color without changing the basic harmonic function of the chord.
Schoenberg opens the chapter on the ninth chord in his Harmonielehre by calling it “the stepchild of the system.” He notes that Schenker did not consider the ninth chord a viable harmonic entity principally on the grounds that it could not be inverted; Schoenberg dismisses this as a “silly hindrance,” and proceeds to demonstrate that it can in fact be inverted and that it can follow the same basic resolution procedures as the seventh chord. He further uses this observation as the grounds for a brief philosophical discussion on the relationship between theory and practice in music.