Hoot - Well since everyone seems to be content to call it a 9 string... *g* However, I (for one) read the title & thought "interesting", only to find that what was meant was a weirdly strung bog-standard guitar. Which wasn't anywhere near as interesting!

There seems to a lot of experimenting going on these days. Double neck electrics have been around for a long time. Five string electric basses are almost commonplace. I wonder how much of it is driven by music & how much by pretentiousness, people looking for an easy way of standing out. I remember reading a blurb for a band not very long ago who were busy boasting how innovative they were, writing a lot of their stuff in 5/4 & 7/4 & other such weird timings. Nope, not innovative, lads, just pretentious. It's generally jarring on the ear & unmusical as a consequence. Useful as a special effect, or for an occasional piece, but tedious when overdone (like strobes in a light show).

The problem, as far as adding courses to a guitar type instrument goes, is that it is a polyphonic instrument. Several notes at a time. How you gonna hold 'em down properly as the neck gets ever wider (never mind that you're running out of fingers)? I've yet to come across a lute piece that involves fretting anything beyond the 6th course (though I'm not suggesting they don't exist). On the really big lutes (anything up to 19 courses, see here), most of the drones don't even have a fretboard underneath!

As to 5 string violins/double basses, they're essentially monophonic (OK, the violin is double stopped occasionally...), so I can see some point to the extra string, but I can't imagine they're much use in an orchestra, frex.