Reference
Ninth Chords
A ninth chord is created by adding a ninth to a seventh chord.
The most common ninth chords are built on the dominant degree in minor keys as well as in major keys. Since the ninth formed is major in major keys and minor in minor keys, those chords are called major dominant ninths and minor dominant ninths.
Ninth chords may also be built on minor sevenths and major sevenths:
The augmented ninth chord is obtained by adding an interval of augmented ninth to a dominant seventh chord. In many occasions, the ninth interval is simplified enharmonically. In these cases, the chord seems to be a dominant ninth chord with a major and minor third:
