This work examines the harmonic theory of the Austrian music theorist Robert Mayrhofer (1863-1935) as described in the author’s first two treatises, Psychologie des Klanges and Die organische Harmonielehre. In presuming that musical listening involves the visual conceptualization of pitch as points in “tone space,” Mayrhofer’s harmonic theory builds upon the perception of the major third as an essential interval, the n-Strecke, to create a harmonic system consisting of distinct harmonic structures as expansions in tone space. These structures,which Mayrhofer calls cells, delineate various levels of expansion in tone space that characterize the boundaries of tonality. From these structural levels, Mayrhofer develops the concept of expanded tonality that in his view underlies most music composed since Bach and is especially helpful in describing the highly chromatic music of late tonality. Mayhofer thus develops a highly original and controversial theory of harmony from a single musical perception.