On the Walrus Comix message board, someone was solicitng questions about music theory and made the mistake of asking me directly for advice. I perhaps should have made the caveat that I have a BFA in Illustration, but I sallied forth bloviatingly nonetheless. I'm reposting my response here, as they reveal a lot about my entire history of songwriting without explicitly saying much of the kind.
Drew asked, "Gives me [a headache] ... but I gotta try - I'm hearing things in my head that I can't find on the fret board or keys.
Why the fuck is a Major Sixth 9 half steps? Why not call it a Major 9th?
Don't get me started on the order of sharps and flats..."
Someone mentioned the circle of fifths, which I thought was a bit misleading for what he was asking.
And I said: The circle of fifths is a major pain - as far as I can tell, the only instrument you need it on is an accordion (the pegboard is arranged in rising fifths, just to really fuck with your head).
A sixth isn't the sixth half-tone up, it.s the sixth note from the root pitch in a seven note scale (with the eighth note being the root an ocatve up - hence 'octave'). Since it's a major sixth, let's say you're in the key of C. The root note - 1 - is 'C', and then the individual pitches are 2 (D), 3 (E), 4 (F), 5 (G), 6 (A), 7 (B). That's a C major scale. If you played Bb instead of B natural, it'd be a dominant seventh scale. Minor scales, you flat the third and the seventh. Etc. Knowing the names of scales isn't so much use, I think. On the other hand, I'm terrible with scales, so maybe knowing the names is a good idea.
It's really knowing how things relate to each other, and what I like about the minimal music theory I've learned is that it gives me a nice organizational principle. It's breaking one large blob down into a series of smaller, more easily digested fragments.
To clarify: a C major 6th chord is so called because it has the 6th note OF THE SCALE added. In this case, an 'A.' A major chord is the first, third and fifth pitch of the major scale - in C, again, that's C, E, and G. So when you add the 6th/A, it's C, E, G, A.
I find it useful not to think in whole and half steps, but in modes/scales, which is why I rambled a bit in the previous post. Beyond my normal predilection for rambling, that is. Obviously, each scale has its own pattern of whole and half-steps that define which mode it is - a major scale, for example, is whole (D), whole (E), half (F), whole (G), whole (A), whole (B), half (C, again). Those intervals remain unchanged no matter what key you're in, which is why knowing the sixth of a scale is ultimately much more important than knowing the pattern of whole and half intervals. Those are easy to commit to memory and will quickly become rote for you, but finding the special coloration you're looking for in your writing is more related to knowing what makes a C into a C maj 7, or a C dominant 7, for that matter.
To be either extra confusing or extra useful (depending on your POV), you'll find that identical chords will repeat themselves in different scales under different names. An A minor scale is (almost) the same as a C major scale, since the minor scale pattern of whole and half steps gives you whole (B/2), half (C/3), whole (D/4), whole (E/5), whole (F#/6), half (G/7), whole (A/8).* Note that all the pitches from the C major 6th chord are there - C, E, G, A. Only when you play it in the key of A minor, they call it an A minor 7th chord, since the 7th pitch (G) is added relative to the root (A) of the triad (A, C, E) that forms an A minor chord.
Myself, I found that working out that these chords repeat themselves removed about 70% of worrying about theory. There are reasons WHY you call one C major 6th and one A minor 7th, but they're completely esoteric for the purposes of general songwriting or jamming or whatever. In other words, just pick one name for a chord and always use that name. I ALWAYS call the damn thing an Am7, regardless of mode. It's 'wrong,' (or at least bad form) but I'm not writing in a conservatory - I'm fucking around on an acoustic in my kitchen.
If you really want it to be a C6th, tell the bass player to play a C. The bass note quite often is the deciding vote for what chord it's supposed to be. Note the music of Hall and Oates, Joni Mitchell and Billy Joel, all of whom loved throwing a different bass note than the root of the chord to create unique voicings that sound 'jazzier' to our ears. On piano, play a C major chord with your right hand and a D bass note with your left. You've just written your first Joni Mitchell song. Note also that if you use too many of these damn chords, you start writing ersatz Steely Dan songs and maybe need to rethink the whole 'complex chords are better, the 70's RULED!' thing.
There are also reasons why sometimes you call a pitch Bb and other times A#, despite the fact that it's the same damn pitch - but the real reason is that music theory is several thousand years old, and like every other system of human thought, it's just got its barnacles and idiosyncrasies, and you should approach it by taking away what you need (more interesting chords) and ignoring the bits that have no use for you (mnemonic math games like the circle of fifths).
Really, I'm sure that 50% of the above is wrong. Again, I think it has more value as an approach to songwriting than any kind of text on music theory.
*Obviously, this is just one kind of minor scale - I'm guessing the dominant?