How did medieval plainchant use church modes?

Medieval plainchant utilized a system of eight church modes to organize melodic structure, derived from early Christian vocal conventions. These modes are classified by their finalis—the note upon which a melody typically ends—and are organized into four pairs of authentic and plagal modes, known as maneriae.12

In this system:
* Authentic modes span an octave starting and ending on the finalis.21
* Plagal modes (identified by the prefix Hypo-) share the same finalis as their authentic counterparts but typically range from a fourth below to a fifth above that note.21

Each mode is further defined by a recitation tone, or confinalis, which serves as a central pitch for melodic movement. While medieval theorists often applied Greek names to these modes, the system was fundamentally based on the diatonic scale and the tetrachordal principle. To maintain modal purity and avoid undesirable intervals like the tritone, performers sometimes employed musica ficta, adjusting notes through the addition of accidentals to conform to the melodic requirements of the chant.123