Some years ago when 8-string guitars gained popularity, I liked the idea 
of tuning way down to bass-guitar range while keeping the high range of the
high e-string. However I did not like the idea of having 8 freaking strings.

So, I thought, "if I tuned to all-fifths while keeping the high e-string, how
low would the lowest string go?"

It turns out it goes down to F, a half-step lower than an 8-string in standard
tuning. By the way, "all-fifths" is like a violin/viola/cello in which the strings
are tuned a perfect fifth apart.

For example, the violin is tuned to g-d-a-e. Its slightly beefier cousin the viola
is tuned to c-g-d-a, going a fifth below the violin. The cello also goes with 
C-g-d-a, but an octave below the viola -- on par with "drop C" on the guitar. You
could play Godsmack's "Awake" with a cello. That would be pretty humorous.

With this fun idea in my head, I decided to test it out. I bought a baritone electric
guitar: the 27"-scale ESP FB-200 from some dude on Craigslist. Beautiful. It was tuned 
to B-standard (the default) so I tuned it as follows:

F-C-g-d-a-e

putting the last string at a high e where I want it. Of course, the 52-gauge lowest string 
was way too floppy at F, so I bought a 68 gauge string to put there and used the 52 for the
C string. Groovy. It's still a bit loose. Ernie Ball has a "Baritone Slinky" that starts at
72 or something ridiculous and I'll try that one with my next paycheck. A string snapped the
other day so it's a perfect time to experiment again!

Anyway, after adjusting the intonation and the truss rod (not much on the latter), I have a
fantabulous range that actually exceeds an 8-string by a half step. Not bad!

But all the chords are different...so what? I don't play folk songs. The basics are there:
major/minor chords, I can easily add a 4th or a 6th, diminished chords, and that fun one that's
a fifth-and-then-a-fifth (maybe a sus2 or something?). Metal!

Lastly of course, the modes are different. That won't be hard to learn though. Some time ago I
found online a symmetric pattern that makes navigating the fretboard easy. Since then I haven't
been able to find the website...perhaps it got taken down. But this symmetric pattern is 
centered on the Dorian mode. It's awesome.