If you lead a busy schedule, a chord is a set of three or more notes. If you have time on your hands, you can worry over whether two notes constitute a chord. :-)
Here are two of the many ways to use it.
- Constructing a chord from the ground up. Suppose you wanted to construct an Am7. The table tells us we need a 1 and a 5 along with a flatted 3 and flatted 7. The A scale has three sharps and is
A, B, C#, D, E, F#, G#, AThen (1,3b,5,7b) becomes (A,C,E,G)...the open strings!
(remember whole, whole, half, whole, whole, whole, half).
- Modifying a chord. We know that the A shaped C chord is (5543) or (C,E,G,C). Suppose we needed a Cmaj7, that is, a Cmaj7. The chart tells us to add the 7-th note in the scale, B. We've got two Cs-that's one to spare--while the 2nd fret of first string is...a B! So, Cmaj7 is (5432)=(C,E,G,B). The usual first postition Cmaj7 one finds in chord charts is (0002)=(G,C,E,B).