Watch the Third Place online guitar lesson by Brad Carlton from Chord Studies: Double-Stops Vol. 1
This double stop study involves the interval of a third, which simply means we're playing a note from the scale, skipping the next note, and then hitting the note after that. We're in the key of A, and our first double stop is A and C♯. We move both of those notes to the next note in the scale (A moves to B, C# moves to D) giving us a Bm chord over an A bass. Then we have D and F♯ outlining a D/A, and we resolve back to an A chord.