classical? i believe.. isn't it all the same anyhow?
the classical (figured bass) and jazz roman numeral system is very different.
1st off, most jazz systems dont use lower case roman numerals at all. in classical, if the numeral is lower case, its a minor chord.
in jazz, "III" always means "chord built on the major 3rd of the key youre in". in classical, however, "III" usually (always?) means "major chord with root on the minor third of the key youre in". in classical, "III" chords usually occour in minor keys. the equivalent chord in jazz notation to a classical "III" chord is called "bIIImaj"
so, if you hear me talking about a "bVImaj7" chord in one of my posts, i am referring to a major seventh chord, having its root on the minor sixth of the key. if i remember correctly, the equivalent chord in classical notation is called "VI7". can you see how this can become confusing (in jazz, a "VI7" chord is something entirely different)?
the way you can tell if your talking jazz or classical is if your using lower case roman numerals (eg.: ii, always classical), and if your saying if things are major or minor (eg.: IImin, always jazz).
its very important not to mix up the two systems, because it becomes impossible for other people to know what you mean.
when a chord is abbreviated as bIV or, biii, does that just mean that you play the IV or iii chord, but with the root flatted?
basically, in classical, yes.
as i just explained, however, in jazz, no.
in jazz notation, you build the entire chord based on the scale degree indicated, and all roman numerals are ALWAYS based on the major scale.
p.s. i have never in my life heard of a bIV chord.
also, what exactly is an augmented 6 chord?
its a classical thing. im a little rusty, im sure someone else around here will know.
(in jazz, you could have and augmented triad with a major 6th on it, but thats entirely different. i think that would be called an augmented 6 chord and would be written: C6aug ? or C6#5. they dont happen very often, it would basically be an inversion of a minor major seven chord)
and lastly, is there any difference between II and V/V or III and V/VI?
in jazz theory, a II7 chord is different than a V7/V because it doesnt go to the V.
eg.:
C7 `` D7 `` C7
I7 `` II7 `` I7
eg.:
C7 ` D7 ``` G7 ` C7
I7 ` V7/V `` V7 ` I7
capisch?