> You're using "harmonic" here in a way this is disconnected from a lot of the rest of the discussion here. We are not talking about scale degrees or anything like that, but the physical harmonic series.

This is such a confused statement. I'm talking about the physical harmonic series, and OH!! I see the CONFUSION NOW HAHAHA. I do NOT MEAN the 3rd and 4th harmonics that HAPPEN to be on the 5th and 7th frets!!

I meant the actual harmonic series, the 5th and 7th, which is at the 4th fret and in the middle between the 2nd and 3rd frets (and also available at other nodes along the string).

The 5th and 7th HARMONICS in the harmonic series are a 5:7 ratio, which is a tritone!

What this means is:

The tritone is NOT based on the 11th harmonic which is not used in much Western music and is significantly flat. It's more of a 2.75 tone than a tri (3) tone. Yes, it's the closest tritone that is precisely a tritone above some octave of the fundamental, but octaves are a real thing. Saying that 11/8 is a tritone is not more compelling than saying that 7/5 is a tritone. The actual 11th harmonic isn't a tritone, it's three octaves plus an almost-tritone.

The idea of deferring to the harmonic series but sticking strictly to the fundamental as the reference and yet also ignoring octaves is a few layers of common mistake in this topic.

Here's how the 7/5 tritone works: It literally is the major third to the minor 7th of a harmonic 7th chord. In other words, when you play it on two notes of an instrument, you play that tritone, first, it sounds like a tritone because it is one. Second, the two notes are part of a harmonic series that starts a couple octaves and a major third below the notes you are playing. With difference tones on a distorted guitar, you'll actually generate the missing fundamental.

Put another way, when you play 7/5 tritone calling it C and G♭, what you're doing is creating a harmony that is part of an A♭7 chord and you're not playing the A♭. There's no reason that the concept of tritone above C needs to be within a harmonic series starting on C, that's not how this stuff works.

I'm not using letters because it's how I think. I understand the whole system of JI theory and prefer thinking in relations and ratios. I'm using letters so you can follow clearly.

The same issue arises with minor thirds. You could erroneously say that the first minor third in the harmonic series is the 19th harmonic because you are stuck on the idea of a minor third strictly above some octave of the fundamental. But the minor thirds that are prominent and make the harmonic basis for minor third intervals in blended chords are the ratios 6/5 and 7/6, which means the intervals from the fifth to the sixth and from the sixth to the seventh harmonics of the series. Incidentally, the 6/5 minor third might be called the upminor or large-minor, it's wider than 12edo. The 7/6 is septimal minor or small-minor or downminor. It's awesome and bluesy. Neither sounds just like 12edo, and they are different from one another, but there's no doubt that these are minor thirds. Consider the standard premise that the minor third is the difference between the major third and perfect fifth — that's what 6/5 is, 5/4 is the major third, and 5/4 * 6/5 = 3/2. And 7/6 is appropriately described as the minor third that you get from the fifth to the seventh of a harmonically-tuned dominant seventh chord.

> I still see some fairly clear impositions of "worldviews" on some of the material [at xen site]

Yes, I agree with you completely. In fact, I'm fairly critical of aspects of the xen community too. They get too dogmatic or mathy without enough give for psychology and real-world practice. I don't defer blindly to them or agree with the community on everything. But there are some people in the community with real depth of understanding without problematic dogma or cultural bias. Consider that Partch fits in a quirky part of that community effectively, and his works and concepts are valuable but are also a quite incomplete and debatable angle.

> I'm the author of a cross-platform DAW, and one of my goals for the next few years it to get support for arbitrary tuning systems deeply embedded into the software.

YES I KNOW, I'M A PAID MONTHLY SUPPORTER, and THANK YOU THANK YOU!

I am a dedicated Free/Libre/Open advocate who uses and promotes Ardour to students and everyone. And having good support for tuning is a key thing I value in music tools. I'm very excited to hear of your interest! Maybe I should post on the Ardour forum and have a different and more productive engagement than this awkward chat here.