I'm having this discussion with a friend, and he feels that "#9" is "incorrect" and that it should be a "b10". He feels that the "Hendrix" chord, the E7#9, is wrong because he views the #9 as the b3 an octave higher than the root. Besides the "that's the way it is" answer, all I could come up with is that the root triad is major, with a minor 7th making it dominant, and the 9 is an extension; in this case the 9 is raised a half step. So to me it's simply a reference to how the extension is modified, not the root triad. The fact that the chord can be approached improvisationally as either major or minor is irrelevant and co-incidental because, using his line of reasoning, how does one account for a b9? Also, it occurred to me that extensions are most always odd numbered; I've never seen any even numbered extensions. It seems to come back to what the root triad is, ie, we use "#11" when the root triad has a perfect 5th, otherwise, without the extension, the same note an octave lower would be a b5. So I thought I'd share this with you guys to get your insights.