A diatonic interval which traverses 8 steps in a diatonic scale, and covers a pitch distance bounded by 9 nominals inclusively, an octave plus an augmented-2nd.
The augmented-9th became a common feature of jazz harmony (especially during the bebop period in the late 1940s), blues, and rock, particularly as an "extension" on top of a dominant-7th chord. This particular chord was used so prominently by Jimi Hendrix that some musicians refer to it as the "Hendrix Chord".
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