To understand chords you first need some knowledge of scales. Think of the old 'doh rae, me' that you learnt in primary school. This is a scale! Depending on where you pitch the 'doh' you are in a higher or lower key. In music notes have letter names so....
The notes of the C scale are numbered 1 to 7 and then start repeating. In C all the notes are natural - think of white keys on a piano. In other keys some of the notes become sharp or flat (piano black keys). For the moment don't worry about why. If we repeat the exercise in G we have to use F# but the rest remains the same.
In D we have two sharps F# and C#
And in A this becomes three sharps - F#, C# and G#
Now to make a CHORD we take a note - any note and add the notes 3 and 5 above it! So Look back at the C table. If C =1 then E = 3 and G = 5. So the notes in a C major chord are C, E, G. In a G chord 1,3,5 gives us G, B, D (loop around again) and in a D chord D, F# and A. When we finger chord shapes on the guitar we are changing the notes on the strings to match these - depending on which chord we want to play. Normally a guitar is tuned E A D G B E (Easter Bunnies Get Drunk At Easter). If those notes are in the chord we want then they can be left 'open' otherwise they have to be altered. Taking G as an example this time,
We can only use G, B, D so..
E is no good - make it G.
A is no good so make it B.
D,G,B are all ok so leave open
and E again becomes G.
This gives us the G Major chord shape:
Now musically speaking the chords formed on different note numbers are different types. Some are Major(M) and some are minor(m). Those formed on notes 1,4 and 5 are Major and those on 2,3 and 6 are minor. We tend not to use note 7.
So what does all this mean?! If you play in the key of G (G is 'doh') you have one sharp note (F#) and the chords you use (and the notes making them) are:
Many songs only need the three majors - that is chords 1,4 and 5 - or G, C and D in this example. Indeed you could play about 90% of folk songs using just these. To add some colour or sophistication we add some of the minors too.
As I said above, Major chords are built on notes 1, 3 and 5 from any given note. For a minor chord we flatten the 3rd by one semitone (one fret on the guitar). So looking above, chord 2, in the key of G, is an Am. The A scale is here :
So notes 1, 3 and 5 would be A C# and E but remember we flatten the 3rd, so this becomes C natural or simply C so the A minor chord is A, C and E.
We can only use A C and E so..
E is ok so leave open.
A is ok so leave open
D is no good so make E
G is no good so make A
B is no good so make C
E is ok so leave open
This gives us the A minor chord shape:
At the last workshop some of you asked about the sus(pended) 4 chords too! I have included those below for reference. In this case the third note is replaced by the fourth so the chord formula is 1, 4, 5.