A diatonic interval which traverses 8 steps in a diatonic scale, and covers a pitch distance bounded by 9 nominals inclusively, an octave plus a minor-2nd.
The minor-9th became a prominent feature of the music of Schoenberg and his pupils (the "Second Viennese School") in the early 1900s, as a substitute for the octave. It then became a common feature of blues and jazz harmony, especially during the bebop period in the late 1940s, particularly as an "extension" on top of a dominant-7th chord -- it had been used in this fashion quite extensively by Beethoven more than a century earlier.
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