Over the past few years, I have made quite a few posts on the history of the dominant major-ninth chord in treatises and textbooks. I have now gathered those posts into a single PDF essay and have provided a small amount of additional material. You can find the PDF here: link.
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I have published an updated index to this blog and to essays about scale degree ^6 and the dominant ninth chord on the Texas ScholarWorks platform. Here is the link. The date for this was November 5, 2024. I have made several posts since then; please see individual blog entries for those.
For a list of all the essays I've posted to Texas ScholarWorks, go to the front page of the UT Faculty/Researcher Works section: link. If you click on Filters/Author, my name will come up as one of the most frequent contributors.