Ab7#9

Construction

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

The 3 and b7 form the tritone (the interval of six half steps) that drives dominant function. The 5 grounds the chord. The #9 is enharmonically a b3 (same pitch class, different spelling), so this chord contains both a major 3rd and a minor 3rd simultaneously. Dom7 with an added #9 -- that built-in major/minor clash is the chord's defining feature.

Harmonic Function

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

• V7#9 -- altered dominant resolving to either major or minor. The #9's ambiguity means it does not commit to one destination the way 7b9 commits to minor.
• I7#9 -- the "Hendrix chord," used as a tonic in blues and rock. In blues, dominant chords do not follow the classical rule of needing to resolve -- they sit on the tonic and the tension becomes the color, not a problem to solve. No resolution needed; the clash is the point.
• Altered dominant -- pairs with the altered scale (seventh mode of melodic minor) in jazz ii-V-I progressions. The altered scale contains every note of this chord (including the #9 as its b3), making it the natural choice for melodic material over V7#9.

Character

Aggressive and ambiguous. As a member of the altered dominant family, the simultaneous major and minor 3rds produce a growling, crunchy sound -- this is the chord behind Jimi Hendrix's "Purple Haze." Compare to 7b9: both alter the 9th of dom7, but 7b9 is dark and pulls squarely toward minor while 7#9 is aggressive and tonally ambivalent, working equally well as dominant or tonic. The chord does not ask to resolve; it asserts its presence.

Related Sounds

These chords share dominant function -- each shifts one color:

• 7 (1-3-5-b7) -- the parent chord, without the #9's major/minor clash.
• 7b9 (1-3-5-b7-b9) -- b9 instead, dark and minor-directed rather than aggressive and ambiguous.
• min7 (1-b3-5-b7) -- remove the major 3rd from 7#9 and the #9 becomes the b3 of min7; the two chords share a hidden kinship.
• 7(#5,#9) (1-3-#5-b7-#9) -- adds a raised 5th, fully altered dominant at maximum intensity.
• Pairs with the altered scale (seventh mode of melodic minor) in jazz, or the blues scale in rock contexts.

Voice Leading

Voice leading tracks how individual notes move from one chord to the next. The #9 can resolve in multiple directions, giving this chord flexibility that more directional altered dominants lack.

• V7#9 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 #9 moves up a whole step to the root of I. The 5 can hold as a common tone or step down to the root.
• V7#9 to i7: The 3 of V moves up a half step to the root of i. The b7 moves down to the b3 of i. The 5 moves to the b3 or root. The #9 holds as a common tone -- it becomes the b7 of i7. This common-tone connection makes the minor resolution smooth.
• I7#9 to IV7: In a blues, no traditional resolution occurs. The #9 is color, not tension. The chord sits on I, and the move to IV simply shifts the same quality up a fourth.

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

Practice Seeds

Hear the clash. Play the major 3rd and #9 together, slowly. Hear both major and minor at once -- this simultaneous presence of both is the chord's identity and what separates it from every other dominant.

Hendrix context. Play I7#9 to IV7#9 in a blues rhythm. Feel the chord as a tonic, not a dominant -- this is its home in rock and blues, where the dominant sound IS the resting place.

Resolution both ways. Play V7#9 resolving to major I, then to i7. Discover how the chord serves both: the #9 moves up to the root for major, or holds as the b7 for minor. The #9's flexibility is the key.

b9 vs. #9. Play V7b9 then V7#9 on the same root. Distinguish dark and directional (b9 toward minor) from aggressive and ambiguous (#9 toward either) -- this comparison defines the altered ninth family.