Of more relevance to the guitar player are Chord Families, or the groupings of chords that go with the keys. Looking again at the C major scale, with the numbered notes we have:
Each note can represent a specific chord in the key of C. For instance, the first note ‘C’ represents the first chord C (major). The fourth note ‘F’ represents the fourth chord F (major). Some of the numbered notes represent major chords, some minor, but the seventh represents a ‘diminished’ chord. Look at this chart of Chords for the Key of C:
Notice that for chords, we typically switch our numbers to Roman Numerals: uppercase for major chords, lower case for minor chords. So we see that the major chords in the key of C are chords (I, IV and V): C, F, and G. The minor chords (ii, iii, and vi) are: Dm, Em, and Am. From this point on I will omit the seventh chord, the diminished chord. I call this the C Chord Family.
For the key of G (and any other key) we can apply the same approach to find our major and minor chords:
Thus the G Chord Family comprises these major chords (I, IV and V): G, C, and D. And these minor chords (ii, iii, and vi): Am, Bm, and Em. For a full chart of these Chord Families, click here: Chord Families. Look at the graphics below that show common chord families that beginning guitarists use (click to enlarge).
Click here to download a PDF version of the above diagram: Chord Family Diagrams
Nashville Number System
One important note to make here is the Nashville Number System. The Nashville Number System does away with Roman Numerals and uses regular numbers to represent chords. This makes it easy when chording a song according to the numbers rather than to a specific key, which is sometimes useful if you know the song may be transposed in the future.
In the key of G a 1-5-6-4 progression would be G-D-Em-C. We know that the 6 chord is minor. If, however a minor chord is instead to be played as a major we can notate it with a capital ‘M’ besides.