An overview of basic music theory

All songs are played in a key. Most songs for most styles can be played using only 8 chords for accompaniment and 6 scales for melodies or solos in each key.
Ron Greene Music Dials instantly show you the chords & scales you need in each key.

The eight chords in each key
you should know  

The 6 common chords come from the major scale notes in each key and are called scale tone chords (I = do, IIm = re, IIIm = mi, IV = fa,
V = so, VIm = la).

The 2 optional chords (bIII, bVII), commonly used for rock & blues, come from the flatted third and flatted seventh (notes of the major scale and can also be used to play songs in each key.)

Except for jazz, most songs use only major, minor and seventh chords and these are the basic chords you should learn in each key. Experiment with and have fun playing, creating or improvising great sounding chord progressions in each key.

Additional Chord Theory

There are four chord types: major, minor, augmented, diminished. There are many extensions of these chord types including: suspended, fifth, sixth, seventh, ninth,
eleventh and thirteenth chords.

Chord formulas define which major scale notes are contained in the chord (1 = do, 2 = re,
3 = mi, 4 = fa, 5 = so, 6 = la, 7 = ti).
Below are the formulas for the 4 chord types and their extensions: 

Additional Scale Theory

Scale formulas define which major scale notes are contained in the scale (1 = do, 2 = re,
3 = mi, 4 = fa, 5 = so, 6 = la, 7 = ti).

The chart on the right shows the formulas for the 6 scales.