Gregorian modes were used during the Middle Ages and the Renaissance. During the Renaissance, they progressively became our major and minor scales. The number of modes varies according to the time period and the theorist, but in general 12 Gregorian modes have been identified.
Gregorian modes have a final, a note with which the melody ends. Its function was similar to that of the tonic in the major and minor scales. Furthermore, Gregorian modes have a dominant. The dominant is a note upon which there is much insistence throughout the melody.
Modes are divided into two categories: authentic modes and plagal modes. Each plagal mode is associated with an authentic mode. Both have the same notes and the same Final. The difference between an authentic mode and its related plagal lies in the nature of the dominant note and in the range or ambitus.
Treatises on Gregorian Chant assign the odd numbers I, III, V, and VII to authentic modes. Their related plagal modes are assigned the even numbers, II, IV, VI, and VIII. The related plagal mode of the authentic mode I is II, that of III is IV, etc.
Some theorists use Greek names such as Dorian, Phrygian, Lydian, and Mixolydian to refer to the authentic modes I, III, V, and VII respectively. As for plagal modes, the prefix hypo is added to the name of its authentic relative: mode II becomes the Hypodorian mode, mode IV the Hypophrygian one, etc.
These modes were forgotten for several centuries. However, variants have come back and have been used again in Classical music as well as in Jazz.
The twelve Gregorian modes are listed below. Finals are indicated with the letter F, dominants with the letter D: