ALTHOUGH THOUSANDS OF SONGS USE ONLY THREE CHORDS (I, IV, and V), there are of course many other chords common to any key. The next most common chords can be found in the harmonized scale—the chords based on the notes of a scale.
Look at the notes of a C major scale: C D E F G A B C
Assign a Roman numeral to each note in the scale: C is I, D is II, E is III, F is IV, G is V, A is VI, B is VII, and then you are back to C.
Build a chord on each of these notes by adding notes a third and a fifth above each of these scale tones. Use only the notes from the C major scale. (Chapter 8, “Chord Construction: How to Build Chords and ...