His harmonic idiom, initially permeated by suggestions of Franck and Scriabin, underwent a considerable refinement during the 1930s, the mature practice coming to be founded on the overtone series. The most important intervals are the perfect 4th and 5th, which Harris referred to as "organum" sonorities, in reference to their historical usage, while the principal chord types are the major and minor triads and seventh-chords (which he often seemed to regard as being generated by interlocking 5ths). From the mid40s on, he generally used triads as building blocks in the erection of polychordal structures.
Harris also developed an overtone-based classification of polychords within a harmonic "color" spectrum extending from "savage dark" to "savage bright," the harmonies being placed on his spectrum according to the degrees to which the notes of the upper triad of each polychord are reinforced by the overtones of the bottom note of the lower triad. He used the series in a similar manner to rank by color the three positions of the simple triad.
His chord movement is founded on an extended set of relationships derived from the dominant and subdominant sides of a given harmony through common-tone connections between chords in these areas. Root movement is frequently by 3rds, though 4ths and 5ths are found at cadences. In his handling of the cadence, Harris recognized its time-honored function in delineating phrase structure and its capability of contributing to both tension and relaxation; consequently, he often reserved his most intense and felicitous harmonic effects for this point, particularly when it served as the final cadence of a work or a of major subdivision thereof.
Harris conceived of harmony as having three specific functions, the details of which he seems never to have elaborated fully: 1) in the architecture of tonalities, 2) for the delineation of melodies, 3) for dynamic resonance. He also believed that "harmony should represent what is in the melody, without being enslaved by the tonality in which the melody lies; at the same time, harmony should center around a tonality sufficiently to indicate that tonality, because tonality is absolutely essential to form, and to harmonic contrast" The first of the above functions seems related to his frequent practice of erecting a section of a composition (sometimes an entire piece) over a harmonic design, which he sketched out first. The second appears to be what he is referring to in his above-quoted remark about the relationship between harmony and melody: one often finds in his works a curious tension involving support and contrast between the harmonic implications of the melody, with its prevailing modal mixtures, and the supporting chords. The third function concerns the voicing, registration, and density of chords.
