Ninth chord
In music theory, a ninth chord is a chord that encompasses the interval of a ninth when arranged in close position with the root in the bass.
The ninth chord and its inversions exist today, or at least they can exist. The pupil will easily find examples in the literature [such as Schoenberg's Verklärte Nacht and Strauss's opera Salome]. It is not necessary to set up special laws for its treatment. If one wants to be careful, one will be able to use the laws that pertain to the seventh chords: that is, dissonances resolve by step downward, the root leaps a fourth upward.
— Arnold Schoenberg (1948)
Heinrich Schenker and also Nikolai Rimsky-Korsakov allowed the substitution of the dominant seventh, leading-tone, and leading tone half-diminished seventh chords, but rejected the concept of a ninth chord on the basis that only that on the fifth scale degree (V9) was admitted and that inversion was not allowed of the ninth chord.