What are the 7#9 chords in jazz?
This chord has many names: "the Hendrix chord", "the Purple Haze chord" or "the Gretty chord". Whatever name you hear, the chord that you should think of is the dominant 7#9 chord.
It has what some theorists call a "funky" or "bluesy" feel, to which some other theorists disagree by calling it a "jazzy" feel rather than bluesy. We'll let you decide for yourself after explaining the chord in more detail. It mainly functions as a turnaround chord (look at our post explaining turnarounds) before returning to the tonic, so many genres, including jazz, blues and rock, have borrowed it for their own uses.
an F7#9 chord on piano
To constitute the chord, it is 1-3-5-7-#9. An example would be F7#9, with the notes F-A-C-D#-G#. It is the addition of #9 that gives the chord its unrestful feeling because the #9 is actually the minor third of the root. When this combines with the major third also present in the chord, you get a chord dying to resolve to the tonic.
In jazz, 7#9 chords, along with 7b9, are used to create the dominant chord (V) in a minor II-V-I turnaround. The chord can be traced as far back as to the 1940s. But unlike in jazz, where you can classify chords according to their chordal degrees to free the possibilities, this chord creates some confusion because the augmented third between the 7th and the #9th degrees clashes with the tertian chord theory, which sees chords as stacked minor and major thirds. It's no surprise that Paul McCartney, who has some responsibility for the popularization of the chord, called it the "great ham-fisted jazz chord".