Chords within a Key (using the Key of G as an example)

Note the Roman numerals! Example: I vi IV V = G Em C D

The G scale is composed of these chords:

I=G ii=Am iii=Bm IV=C V=D vi=Em viio=F#o (then back to G).

(These chords may be played in any order.)

"Let It Be" by The Beatles is an example, below (I-V-vi-IV).