OK , here are some fundaes on how guitar chords are formed ! These are some lessons I learnt from the guitar classes I take over here in Calcutta .

The formation of Major Chords are absolutely no problem...there are only two rules to remember.

OK I believe it will be easier to illustrate straight away with an example. Let us start with the C-Major chord.

We start with the C note and arrange all notes thereafter, such that

the third and the seventh notes are semi-tones and all others are full tones.

So in the case of the C-Major scale the notes are

C

D

E

F

G

A

B

C

Note that the notes are obtained by taking the notes from the folllowing set of complete notes ,according to the rule specified

C, C# , D , D# , E , F , F# , G , G# , A , A# , B , C

Now that we have the table, all we do is take the first ,third and fifth notes in the table to form the notes for the C-Major chord.

In this case it is  C,E and G. So it is about as simple as that ...

Now we can try the B-Major chord.

Proceeding in a similar fashion, we obtain the following table.

B

C#

D#

E

F#

G#

A#

B

Now taking the first, third and fifth notes we have

B, D# and F#.

This forms the notes for the B-Major chord.

Since there are two sharps in the chord, the B-Major chord is also called a two-sharp chord.

Anyway , once you have formed all the Major Chords the rule for forming the sevenths are simple, it involves taking the seventh note in the table that you have formed.

So in the case of the C-7 chord, the notes will be

C,E,G and B ...

Simple when you know how isn't it :)