Common Chord Progression

We have been studying the sound and construction of the G Major Scale.

If you take it a step further and build chords on each of the scale notes you get something that looks like this:

The big chords in this key are the primary chords (I, IV, and V7) G, C, and D7. These are the chords that outline the key and are used in hundreds and hundreds of songs and a entire genre of music (The Blues). 

Another chord progression that is incredibly common is the chord progression 

G, Em, Am, D7    (I, vi, ii, V7) . This chord progression makes use of primary (I, IV, V7) AND secondary chords (ii, iii, vi, vii). Like the I, IV, V7 progression, this progression has been used in hundreds of songs and will probably sound familiar to you as you build up clarity and smooth transitions between chords.