The A Dominant 9th chord is a five-note chord made up of A, C♯, E, G, and B. It is built from a root, major third, perfect fifth, minor seventh, and major ninth.
Construction
A Dominant 9th = Root + Major 3rd + Perfect 5th + Minor 7th + Major 2nd = A · C♯ · E · G · B
| Note | Interval | Degree |
|---|---|---|
| A | Root | 1 |
| C♯ | Major 3rd | 3 |
| E | Perfect 5th | 5 |
| G | Minor 7th | ♭7 |
| B | Major 2nd | 9 |
Key Signature
A dominant chord points home to the key a fifth below its root: the A Dominant 9th is the V (dominant) of D Major, so the relevant key signature is that key’s — 2 sharps (F♯, C♯). Spelled as a scale, these notes are A Mixolydian.
F♯C♯
Order of sharps
Sharps are added to a key signature in a fixed order. Each new sharp key adds the next sharp on the list.
F♯C♯G♯D♯A♯E♯B♯
Mnemonic: Father Charles Goes Down And Ends Battle
Chords in the Key of D Major
These are the triads built on each degree of the D major scale:
A Dominant 9th — Frequently Asked Questions
What is the A Dominant 9th chord on piano?
The A Dominant 9th chord contains the notes A – C♯ – E – G – B. On piano, play these notes together to sound the chord.
What notes are in the A Dominant 9th chord?
The A Dominant 9th chord (A9) contains 5 notes: A, C#, E, G, B. Formula: R-M3-P5-m7-M9.
How is A9 used in music?
A9 is used in jazz, fusion, and contemporary music to add harmonic color. It appears as a dominant or tonic chord depending on context.
What is the scale degree formula for A9?
A9 uses scale degrees 1-3-5-b7-9, giving it its distinctive sound.