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These Drop D power chords can be played as a 3 string bar chord (string 6-5-4) using the index, the 2, the 2 & 3, the 3, or the thumb over the top of the board. For teaching groups, using the thumb is one of the fastest ways to get results.
Get into Drop D
Doublestops on the open strings. GB together. Think like those are the 3 & 2 open in tablature. Open 3 & 2 together looks like what you see above.