The Theory Behind Chord Families

I have spoken in class about how certain chords belong together because they are in the same family. Well, by family I am talking about the key in which these chords reside.
 Let's look at three chords: G, C, and D

 

These three chords get played together a lot. We call these chords the I (G), IV (C) and V (D) in the key of G.

When you look at the Key of G and how the scale in that key is constructed, you will see the pattern of chords that live within the key of G. This pattern is the same in every Major scale (Do, Re, Mi...etc.)

Notice how the I, IV and V are upper case or Major chords. We call those the Primary Chords in the key. Hundreds and hundreds of songs have been written based on these primary chords alone. At this point, you already know the primary chords and a couple of secondary chords (Am, Em) in the key of G so you are well on your way to playing, and creating some great music.