Collection. The four-note harmony, D-E-G-A, can be thought of as a segment of the circle of fifths, G-D-A-E, and thus also a segment of four possible diatonic collections and two possible pentatonic collections. Within that stack of perfect fifths, it is the central one (D-A) that is filled in melodically and harmonically. The other two fifths are arranged symmetrically around that central one.