Watch the Melodic Motions online guitar lesson by Brad Carlton from Chord Studies: i to V7 Vamps Vol. 1
This progression is a I-IV progression in the key of G. It is using the G and C chord, but we have got a moving line in the third voice. Notice in the G5 chord we are muting both E strings, and we begin our melody with the open D. That voicing then moves up to an E with our Cadd9. Now our bass note goes back to G and we have the natural 7th with F♯ on the D string carrying that melodic movement. For the last chord we return back to the Cadd9.