Sometime in the 1970s, when I was doing a lot of early music, I had the idea for an instrument that was suited to playing Renaissance vocal music. The instrument would be like a guitar, but the pitches of the strings would be closer together. There would be several advantages to doing this:
The last of these ideas was what got me the most excited; it meant that for the music I had in mind (which typically had few sharps or flats), the notation became almost like a tablature: when a note was on a line (and was not inflected by an accidental), it meant that it could be played on an open string; if it fell on a space (or was inflected by an accidental), it meant you'd need to finger it.
My thought was that the instrument would have as many strings as possible, and that the strings would be shorter than those on the guitar, since the focus would be on the first few frets. Renaissance vocal music seldom went below low C (two octaves below middle C), and that was about as low as a guitar could work (especially with shorter strings), so I made that the lowest string. Twelve seemed like a good number of strings, which meant the pitches would be:
The blue notes show the lowest and highest strings on a normal guitar.
Here's how the fingerboard would look, more or less:
I worked out the fingerings for the major scales ...
... and for the opening of the first fugue in J. S. Bach's Art of Fugue:
Since I didn't have the money to hire a luthier to make me an instrument, I experimented with a pair of guitars I had. On one, I added a seventh string and tuned it to the top strings; the other, I tuned to the lower six strings. I couldn't use this arrangement to find out what it would be like to have the full 12-string instrument, but there were some other things that I learned. For one thing, the 7-string treble instrument worked very differently from a normal guitar; having one more note per octave might not seem like a lot, but the density of pitches gave it quite a different sound. It also made it much easier to do extended jazz harmonies. Also, I found that having the pitches C E G B D F as open strings that were low enough to work as bass strings, meant that I could use them like unfretted theorbo strings.
I got very excited when I saw Narciso Yepes' 10-string guitar, and other people have built guitars with lots of strings; here's one with eleven strings (by Rodolfo Cucculelli):
So it is possible ...