r HIS PAPER PRESENTS an analysis of Johnston's extended just harmonic structures as organized by serial processes in String Quartet No. 6, and includes a discussion of the microtonal intervals used and their realization. Without significant familiarity with Johnston's system of just intonation and the associated microtonal notation, both the theoretical understanding and the performance of this quartet are likely to be thwarted. Theoretical understanding encounters a formidable barrier in the composer's complex system of microtonal notation and just intonation. For the same reasons, performers might find themselves forced to gloss over the challenge of realizing the microtonal inflections. This paper seeks to present a map of the interrelated scalar, harmonic, and serial structures that is sufficiently detailed to aid both the understanding and the performance of this composition.