Major Chords
------------
A major chord is made up of the 1st (root), 3rd and 5th degree of a major scale.  
The chord C major, for instance, is derived from the 1st, 3rd and 5th notes in
the C major scale. The C major scale consists of the notes C, D, E, F, G, A, and
B.  Therefore a C major chord would consist of the notes C, E and G.  Various
forms of the C chord are illustrated below.  The numbers to the right of each
diagram indicate the fret location.

Cmaj, C form    Cmaj, A form    Cmaj, G form    Cmaj, E form    Cmaj, D form
===========     +-+-+-+-+-+     +-+-+-+-+-+     +-+-+-+-+-+     +-+-+-+-+-+
| | | | 1 | -1  1 1 | | | 1 -3  | | 1 1 1 | -5  1 1 | | 1 1 -8  | | | 1 | 1 -12
+-+-+-+-+-+     +-+-+-+-+-+     +-+-+-+-+-+     +-+-+-+-+-+     +-+-+-+-+-+
| | 2 | | |     | | | | | |     | | | | | |     | | | 2 | |     | | | | 2 |
+-+-+-+-+-+     +-+-+-+-+-+     +-+-+-+-+-+     +-+-+-+-+-+     +-+-+-+-+-+
4 3 | | | |     | | 4 4 4 |     | 2 | | | |     | 3 4 | | |     | | 3 | | |
+-+-+-+-+-+     +-+-+-+-+-+     +-+-+-+-+-+     +-+-+-+-+-+     +-+-+-+-+-+
| | | | | |     | | | | | |     3 | | | | 4     | | | | | |     x 4 | | | |
+-+-+-+-+-+     +-+-+-+-+-+     +-+-+-+-+-+     +-+-+-+-+-+     +-+-+-+-+-+
===========     ===========     ===========     ===========     ===========
G C E G C E     G C G C E G     C E G C E C     C G C E G C     x C E G C E
5 1 3 5 1 3     5 1 5 1 3 5     1 3 5 1 3 1     1 5 1 3 5 1     x 1 3 5 1 3

Above each diagram is the open chord form that the chord is formed from.  For
example, the 'A form' C chord (2nd diagram) is really the open A chord moved
down to the 3rd fret.  This is an interesting feature of the guitar as opposed
to piano, the same shape can be moved about on the fretboard, taking on dif-
ferent chord names as you go.  Slide the 'A form' C chord down a fret and you
have a C#, one more and you have a D, one more and it's a D# (or Eb), ...
The are other forms of the C chord, but most of them are derivations or com-
binations of notes from the 5 shown above.  The only requirement is that it 
have the 1st, 3rd and 5th degree of the scale, and nothing else.