Excerpted from Acoustic Blues Guitar Fundamentals.
Let’s begin by entering open G (D G D G B D) from standard tuning. Lower both E strings a step to D and lower your fifth string from A to G. Strings 2, 3, and 4 remain the exact same. This is a crucial thing to bear in mind because it suggests that whatever you already know in standard tuning on those 3 strings– chord shapes, scale patterns– still applies. The standard I, IV, V chord development in G is simple to discover (Example 1) simply by strumming the open G chord and disallowing the fifth fret for the C chord (IV) and the seventh fret for the D chord (V).
However after playing chords this way for a while you’ll probably invite some various voicings. Down near the nut you’ll discover a number of cool chords. Putting your fingers on the fingerboard as if you were playing a routine C-chord shape in basic tuning yields as powerful a chord as you’ll ever hear (Example 2a). This puts the flatted third of the G scale (the seventh of C7) in the bass, laying some serious hot sauce on that chord grip. The D7 displayed in Example 2b isn’t quite as gnarly; the seventh on the 2nd string mixes into the chord a little bit more, and the D notes on the leading and bottom offer it a great ringing, droney sound.