There are four chords in every key which are used more often than any others.

They are called the tonic (I), the sub-dominant (IV), the dominant (V), and the super-tonic chord (ii). Most songs begin and end on the tonic so it functions as "home base". The other 3 chords are used to provide contrast to the tonic. Learn what the four main chords are in every key, and guess what -- you've got the magic key to which chords are likely in a song. You'll also learn to match your melody note to these 4 chords, so you can harmonize virtually any simple song.

"How To Play Roughly Half Of All Songs Using Just Four Chords: I, IV, V, and II"

Instructional Audio CD and DVD video & "Most-Used-Chords" Sheet