It's nice to have this forum for all things theoretical, so I'll do my bit for anyone confused about chords and how they are made up and named.
Relative beginners start here:
Strictly speaking, a chord can be two notes and up. For guitarists a power chord of C5 will be C and G: using a distorted sound on this will sound good, but if you add the 3rd (E) it will muddy up the sound.
Major chords will comprise the major 3rd with the root and 5th:C, E and G. Minor chords will have the flattened 3rd : E flat (Eb)in this case.
Sus chords have the 3rd replaced by the 4th, so a C sus chord will be C F and G. Listen to the Who's mighty 'Pinball Wizard' for this one.
You can add a 6th to either the major or minor chords, which in this case will be A. You'll hear 6th chord in Glenn Miller swing band stuff, and the final chord of the Beatles 'She Loves You'.
A Cmin6 chord will have the notes C Eb G and A. If you flattened the 6th to Ab it changes the whole chord relationship and turns it into an Ab major 7th: don't get confused, I'll come to major 7ths in a minute!
Now to 7ths: there are several versions.
A basic C7 chord will have the notes C E G and Bb, and the 7th is flattened from the major scale. A C major 7th has the B natural at the top, whereas a Cmin 7th will comprise the notes C Eb G and Bb.
For a really dramatic chord, try a C min with a major 7th: C Eb G and B natural. Now try it with some big strings and you've got instant drama!
The last version of a 7th is the diminished 7th: This is made up of minor third intervals: C Eb, F sharp, and A. Counting up throuugh a diminished scale you end up with seven notes, although the top one is a 6th up from the C.
Don't worry about it, it's still a diminished 7th chord.
Now here's an interesting thing: there are only three diminished chords in existence: the chord of Cdim, Eb dim F sharp dim and A dim ALL have the same notes in them, and it'll be the bass note that distinguishes which chord you name it. Same with C sharp, E G and Bb, and D F Ab and B. You can also have an augmented chord made up of major 3ds: C E and Gsharp (Aflat), Try playing the chhhords of C, Caug, C6 and back again over a latin groove and you'll see how this one works.
There's also a half diminished chord, which has the flat 7th instead of the nominal 6th of a diminished chord: C Eb Fsharp (Gb and Bb). You can also call it C7b5.
More advanced chords.
Still with me so far?
Ninth chords will act in the same way as a 7th chord: C9 = C E G Bb and D, C maj 9 is C E G B and D, while Cmin9 is C Eb G Bb and D.
You can flatten the 9 to Db or raise it to D sharp. The latter is the first chord of Cream's 'I feel free' ---although it's an E raised 9 chord (easier on guitar).
11th chords add another note at the top: in this case F. Because F is also the 4th in a scale, it'll clash against the 3rd E, so we leave the E out and have the notes C G Bb D and F. If that seems a bit complicated, try looking at it this way: take the top four notes and you'll see that they make up a Gm7: G Bb (min 3rd), D and F. So another simpler way of looking at an 11th chord is Gm7 over a C bass note (Gm7/C for shorthand).
The final chord is the 13th, which has an A at the top. However, we'll call it a 13th rather than a 6th, as we'll still have the flat 7th underneath.
And strictly speaking, we'll lose the 11th underneath it.
In order to get it to sound in the best way, we'll alter the chord voicing (the order of the notes above the root), so a C13 voicing might be C, G Bb, E and A.
Of course you can add the 11th for a more advanced chord, which would be: C, G, Bb, D, F and A (no third E here). You might want to ditch the G, and the shorthand for this can be Bbmaj7/C.
With the more advanced chords, you can really go to town raising and flattening notes in the chord: Try flat 5s, 9s and sharp 11s and see how they sound.
My all time favourite (which I use a lot) is C 13 aug 11: C, G E Bd D Fsharp and A!
So there it is: I hope that that's helpful. Ask any questions and I'll help if I can!
Dave (chordal anorak)
Lots of Ataris which keep on going, 12 Kurzweil 1000 modules, a bunch of hardware synths. Still recording to tape -the old ways are best.....