A pun on Just Intonation, coined by Robin Perry to describe the fretboard of a guitar he had specially made. (In colloquial English, "just about" means "quite close to, but not exactly".)
The only guitars I've seen that come close to real JI while still using straight frets across all six strings, are the guitars refretted by Robin Perry which he calls 'just-about intonation'.
One guitar Robin lent me has 3 different fret distances: 70, 85, and 115 cents. On any given string, this produces scale-steps that give close approximations to such ratios as 10/9, 7/6, etc.
How does Robin tune his open strings?
To be honest, I'll have to write him and ask... too bad he's not on the List anymore. As a general rule, he tunes the open strings to a JI (or pseudo-JI) chord, but of course any tuning at all can be used for the open strings, including one that more closely resembles the 'regular' tuning.
I wrote a very brief description of this guitar 7 months ago, right after visiting Robin during my road trip west:
me, monz, Yahoo tuning group message 8889 (Sun Feb 27, 2000 1:22 pm):
It has 3 different sizes of frets which go straight across the neck: 70 cents, 85 cents, and 115 cents, in this repeating pattern:
|| | | | | | | | || 70 | 115 | 85 | 115 | 115 | 85 | 115 | || | | | | | | |
Most of you should be able to see right away some of the just ratios that it approximates well. The tuning of the open strings is completely 'user-definable', but Robin generally uses tunings which produce a nice rich JI chord on the open strings.
To the best of my recollection, he tuned this particular 'Jinto' guitar to a pseudo-JI 'major 9th' chord.......
Yes, I still am on the list, and still, unfortunately, so swamped with L.O.T.T. (life other than tuning) my "reading" on the list is mostly scrolling. My posts, such as they are, are on the skimpy side.
The tunings I used for the Just-about were, as Joe said, basically open chords. From any fret on this guitar, you can move up in either a 1/2/5 or a 1/4/5 type progression and in some cases, both. There are many other chords accessible of course, but you really have to study the relationships to get something worthwhile out of it. The resolution of the just-about is to the nearest five cents (240 TET).
I decided to have the guitar re-fretted the way I did because I felt that the fret spacing in the higher number E.T.s would be beyond me. I wanted something close to 12 TET spacing. I have since re-thought that limitation and want to get hold of a 31. If mandolin players can get used to it, why can't I?
Below is a diagram of the fretboard, in cents, with the open strings tuned to the closest approximation in this tuning to an open 1/1 major-9th chord.