People looking at this should also look at the Circle of Fifths (google away). It shows intervallic relationships between notes and chords, from the most conson

People looking at this should also look at the Circle of Fifths (google away). It shows intervallic relationships between notes and chords, from the most consonant to the most dissonant. Most folk and rock music is built on the Circle of Fifths. For example, for a G, the adjacent notes on the circle are C and D. If you're playing in the key of G, you will probably have C and D chords as well. Millions of songs consist of just G, C, and D. Likewise, A/D/E, C/F/G, etc. So the basic 15 "cowboy chords" are enough to play countless songs in several different keys. More importantly, once you get the sound of Circle of Fifths changes, you'll find that you can easily play most songs entirely by ear, in any key!