This book confronts an enigma of early writings on music: why chant, which was understood to be divinely inspired, needed to be altered in order to work within the then-operative modal system. To unravel this mystery, the book creates a broad framework that moves from Greek harmonic theory to the various stages in the transmission of Roman chant, citing numerous music treatises from the 6th to the 12th century. Out of this examination emerges the central point behind the problem: the tone system advocated by writers coming from the Greek harmonic tradition was not suited to the notation of chant and this basic incompatibility led to the creation of new theoretical constructs. By tracing the path of subsequent adaptation at the nexus of tone system, mode, and notation, the book promises insights into what mode meant to the medieval musician and how the system responded to its inherent limitations. Through an examination of the major musical treatises from the 6th through the 12th centuries, this text establishes a central dichotomy between classical harmonic theory and the practices of the Christian church. The book builds the foundation for a broad and original reinterpretation of the modal system and how it relates to melody, grammar, and notation.