In a tonal song, all of the tones relate to a single key. Relativity of tones is at the heart of tonal music. Relativity is built into finitude itself, as form and limit give rise to systems of interrelatedness. Congruence of interrelated elements is perceived as harmony.
The Trinity, however, infinitely transcends form, limit, and systems of relations. Systems revolve around a single principle which organizes disparate elements into a coherent whole. The elements interrelate as parts of a whole. But the Father, Son and Holy Spirit are not parts; each person is God indivisibly. The Trinity has no “home key.” Persons, by virtue of their absolute diversity, cannot be systematized. Nothing “holds them together” as parts. Persons are “wholes” for lack of a better term; “part” and “whole” fall short as concepts as they are relative and give rise to one another.
Absolute identity (monad) and absolute diversity (triad) are simultaneous, neither having priority over the other—an indivisible circumincession of persons (“wholes”) without parts. The Three One transcends interdependence, interrelations, and relativity, all of which belong to the spatiotemporal domain of parts outside parts.
Since harmony is such a beloved concept, however, we may perhaps say that Trinitarian Love is an ineffable harmony beyond harmony.