How did medieval church modes provide a framework for liturgical melodies?

Medieval church modes provided a structured framework for liturgical melodies by organizing them into a system of eight scalar arrangements, which were categorized into four pairs of authentic and plagal modes. This system was rooted in the principle of the finalis, the foundational note upon which each mode was built.12

The framework functioned through several key elements:

• Classification: Modes were grouped into four maneriae (manners), where each pair shared the same finalis—D, E, F, or G.2
• Authentic vs. Plagal: Authentic modes typically spanned an octave starting from the finalis, while their corresponding plagal partners (prefixed with Hypo-) ranged from a fourth below to a fifth above the finalis.12
• Recitation Tones: Each mode featured a unique tenor or confinalis—often the upper fifth for authentic modes or the third for plagal—which served as a central pitch for melodic recitation.1
• Melodic Formulas: Beyond their scalar structure, the character of these melodies was defined by distinctive melodic formulas, which helped maintain the modal integrity of the chant.2

This system allowed for a consistent, monophonic melodic language that prioritized melodic needs over harmonic ones.1