I'm probably preaching to the choir in a story about guitars, but if you're looking for some good 12-string music, Leo Kottke's 6 and 12 String Guitar is a good start.
If you haven't heard (or seen) Michael Hedges play a harp guitar, there's some background and links to videos here:
The author is Gregg Miner, and Gregg has an extensive library of harp guitar music. A long time ago I met Gregg when one of my friends Brad Hoyt (a keyboard and harp guitarist as well) hosted a harp guitar convention here in Indianapolis, and I got recruited to help. Gregg and the rest of the artists that were there were extremely approachable, and most of the crowd were musicians (some pretty famous ones, too) checking out one of the more interesting variations of the guitar... and what you could do with one. Fun times.
What I love about links like this is that we see information being shared just for the love of it. This person has done a lot of work simply to share knowledge with the world about something they are passionate about. This is what great about the internet and is easily lost amongst all the commercial content.
Just imagine, had this been a Facebook event instead, none of this information would have been available to us outsiders. The open web lives, even if bruised and limping.
In the early days of the guitar it had tied gut frets, it was microtonal by nature and you could move the frets for different temperaments. Then in the 19th century things changed and the guitar got fixed metal frets, ever since people have been trying to figure out how to have the benefits of both. 1852 is the first attempt that I know of.
It is still worth noting that the microtones used by Angine de Poitrine are usually just transient notes, and could instead be played portamento with smooth transitions between the notes (eg on a fretless board) in most situations. It still sounds really cool, though, and certainly a technical and musical accomplishment. But the music itself still follows the usual patterns closely.
I'm a bit late on the reply, so to add to what has been said by AlecSchueler consider the piece I linked above being played on that guitar slung mini harp.
It's a cover of an English prog rock piece, the titular Discipline from 1981 by the band King Crimson with the then line up of a drummer's drummer, Bill Bruford, and three top form guitarists, Robert Fripp (sitting on chair), Tony Levin (Chapman stick), and Adrian Belew [0].
One comment on the linked performance is simply: 3+2/4 vs 2+3/4 vs 9+8/16 vs 4+4+4+5/16 INSANE
So, ... it's an instrument that allowed for a credible attempt at covering three "insane" polymeter prog rhythms by a single performer.
I'm old enough to have got the album when it first came out- looking for a live performance I stumbled over a recent breakdown look back at a peer album track: Frame by Frame [1]
A harp can have more than 6 notes ringing out at a time. It's also a different instrument and if you've trained to play harp then a guitar is useless for you.
You only have to put your finger somewhere in between two frets to get a particular note, so you can play more than the tiny handful of chords that can be played on something like a violin.
There are non-fretted guitars but they are rare, and I think early banjos were not fretted.
I can play chords on a violin or viola and strum them. Keeping a four-finger chord perfectly in tune in rapid changes is really hard (near impossible) without frets.
There are those who say it isn't a chord until you have 3 notes. I'm going to skip the complex music theory behind the arguments (I haven't studied them enough to understand much less form an opinion).
Of course in most music contexts there is more than one instrument playing and so the others can fill in the missing note making a full chord. Only a "one-man-band" really feels the loss.
in college a good friend was a very good Oud player (plucked, some number of strings.... more than 8? I forget), and it definitely sounds different from guitar due to less sustain. Clearer notes and chords, but they don't linger as long or loud.
Multi-course ouds and lutes of course predate anything resembling the modern guitar by a long, long way. Do you consider those in the “guitar family”? If so, I’d refer to the group as oud or lute family. Guitar family is a recent subset of lute family, and doesn’t even contain the first multi-course instruments.
They are as old as we can trace it back, vihuela/viola de mano were multi-course, as was the baroque guitar which developed from them. The romantic guitar brought us single string courses and in the guitar family double courses died out other than in a few nooks and crannies. I don't think this has any effect on what TFA proposes and the author is well aware of this bit of information, but it has been years since I read it and maybe I am forgetting something?
agreed - America went through a "mandolin mania" in ~1880s, and even in the 1900s and 1910s, the guitar was by no means an established instrument.
As a matter of fact, the "tenor guitar" was made for people who knew how to play the very popular four string banjo, but wanted to play a guitar without learning new chords.
Instruments were in a state of change, and the "harp" nomenclature (and styling) for a 12-string guitar was more a marketing flourish than a family lineage.
If you haven't heard (or seen) Michael Hedges play a harp guitar, there's some background and links to videos here:
https://michaelhedges.com/michael-hedges-the-harp-guitar/