A#9

Construction

Built from these intervals: 1-3-5-b7-9.

Dom9 is dom7 with an added 9 -- the second degree of the scale voiced an octave higher. The 3 and b7 still form the tritone (the interval of six half steps) that drives dominant function. The 9 adds brightness and fullness on top of that tension, widening the chord's color without changing its purpose. Because the 9 sits far from the 3 (they are a full octave plus a step apart), it adds color cleanly with no clash.

Harmonic Function

In Roman numeral analysis (uppercase = major, lowercase = minor):

• V9 in any key -- enriched dominant that resolves to I with more color than V7. The tritone resolves identically; the 9 moves by step or holds as a common tone.
• I9 in blues -- tonic ninth, standard blues voicing that extends the I7 tradition with added richness.
• V9 in ii-V-I -- in this foundational jazz progression (ii = minor, V = dominant, I = tonic), the 9 provides voice-leading connections that plain V7 lacks. Reaching for V9 over V7 is the jazz norm, not the exception.

Character

Rich and bright. As a member of the dominant family, dom9 carries the tritone's tension but the 9 adds sparkle -- warmth without softness. This is the default dominant sound in jazz and the enriched tonic in blues. Compare to dom7: dom9 adds only the 9, but that single extension opens the chord from lean and urgent to full and commanding. The tritone still drives resolution; the 9 dresses it up.

Related Sounds

These chords are closely related -- each modifies one interval:

• 7 (1-3-5-b7) -- parent chord; leaner and more direct without the 9's brightness
• add9 (1-3-5-9) -- same 9 but no b7, which removes all dominant tension; a different chord entirely
• 13 (1-3-5-b7-9-13) -- adds the 13 on top of the 9 for maximum richness
• 7b9 (1-3-5-b7-b9) -- lowers the 9 a half step, turning brightness into darkness; strong pull toward minor
• 7#9 (1-3-5-b7-#9) -- raises the 9 (#9 is enharmonically a b3), creating aggressive major-minor clash

Voice Leading

Voice leading tracks how individual notes move from one chord to the next. The 9 provides an extra voice that connects smoothly across chord changes, often holding as a common tone or stepping down.

• V9 to I: The 3 moves up a half step to the root of I, the b7 moves down a half step to the 3 of I -- the tritone resolves as usual. The 9 steps down to the 5 of I.
• ii9 to V9: The 5 of ii9 holds as the 9 of V9 -- a common tone that makes the ii-V connection seamless. The root of ii9 becomes the 5 of V9.
• V9 to Imaj9: The tritone resolves (the 3 rises to the root of I, the b7 falls to the 3 of I). The 9 becomes the 6 of I, or in an Imaj9 voicing, holds as color above the resolution.

These movements apply in any key -- the intervals are the same regardless of root.

Practice Seeds

Dom7 to dom9. Play a dom7, then add the 9 from the same root. Hear how a single note transforms lean tension into full brightness -- this is the extension that separates the two.

ii-V-I with ninths. Play a ii-V-I progression using ninth chords in several keys. Listen for the common tones connecting each chord, especially the 5 of ii holding through as the 9 of V. Then try it from another root to build fluency.

Blues ninths. Play a 12-bar blues using I9, IV9, and V9. Notice how the 9 adds richness to every position without changing the blues character.

9 vs. add9. Play a dom9 and an add9 from the same root. The b7 is the difference -- without it, the dominant tension vanishes entirely. This builds awareness of how the b7 shapes a chord's function.