A Dominant 9th
9
0 - 4 - 7 - 10 - 14Dom7 + major 9th
About A Dominant 9th
A9 is the blues turnaround made comfortable. Root on the A string at the fifth fret, the ninth sitting on top, and a grip that stays the same wherever it goes — this is the chord that separates a shuffle played by a beginner from one played by someone who has listened to records.
The notes
A, C♯, E, G, B. The G is the flat seventh and the B the ninth; the E is normally dropped, because five notes on six strings leaves no room for the hand and the fifth contributes least.
Against A7 the added B is a single note and an entire register of style. A7 belongs to country and folk blues; A9 belongs to soul and jazz, and the distance between them is that one interval.
The shape
Index barring lightly across the middle strings at the fifth fret with the root on the A string. Nothing open, nothing to adjust — the same grip gives every ninth chord on the neck.
Because no string rings freely, the chord depends entirely on the right hand. Struck and released it becomes percussion; held it becomes harmony. Most rhythm playing uses both within a bar.
Where it shows up
As the tonic of a blues in A that wants gloss, and as the V of D major in soul and gospel. In the first role it sits unresolved for twelve bars at a time.
It is also the standard ending chord of a jazz blues: the last bar of a shuffle in A very often lands on A9 rather than A7, because the ninth softens the stop.