A Dominant 9th (A9)
Notes in A Dominant 9th
Inversions
Same notes as A Dominant 9th, with a different chord tone as the lowest note. Useful for smoother movement between chords instead of jumping around the keyboard.
Root position
A in the bass
1st inversion
C# in the bass
2nd inversion
E in the bass
3rd inversion
G in the bass
4th inversion
B in the bass
Open voicings
A close-position chord like A Dominant 9th can sound cluttered in the middle of the keyboard. Jazz players open it up by dropping one note down an octave, spreading the same notes wider without changing the chord.
Drop 2
G on the bottom
Drop 3
E on the bottom
The notes A, C#, E, G and B make up A Dominant 9th.
A dominant seventh chord with a ninth added on top keeps all the pull of the plain dominant while sounding fuller and a little sweeter. Funk, soul and blues use it constantly, often as a straight swap for a plain dominant seventh whenever the arrangement calls for a richer texture.