In medieval music theory, the Latin term modus (meaning "a measure", "standard of measurement", "quantity", "size", "length", or, rendered in English, mode) can be used in a variety of distinct senses. The most commonly used meaning today relates to the organisation of pitch in scales. Other meanings refer to the notation of rhythms.
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In describing the tonality of early music, the term "mode" (or "tone") refers to any of eight sets of pitch intervals that may form a musical scale, representing the tonality of a piece and associated with characteristic melodic shapes (psalm tones) in Gregorian chant. Medieval modes (also called Gregorian mode or church modes) were numbered, either from 1 to 8, or from 1 to 4 (protus, deuterus, tertius, and tetrardus) in authentic/plagal pairs, and sometimes named after the ancient Greek tonoi (with which, however, they are not identical).
| Authentic modes | Plagal modes | ||
|---|---|---|---|
| I. | Dorian | II. | Hypodorian |
| III. | Phrygian | IV. | Hypophrygian |
| V. | Lydian | VI. | Hypolydian |
| VII. | Mixolydian | VIII. | Hypomixolydian |
In the medieval theory of rhythmic organisation, a mode was understood to be a stereotypical sequence of long and short notes, comparable to a foot in poetry. Rhythmic modes were described by the medieval theorists according to a classification numbered from 1 to 6 (of which only 1 to 3 were of practical importance.)
Rhythmic modes were the basis for the notation technique of modal notation, the first system in European music to notate musical rhythms and thereby make the notation of complex polyphonic music possible, which was devised around 1200 AD and later superseded by the more complex mensural notation. Modal notation indicated modes by grouping notes together in ligatures.
In the notation system of mensural notation (after c.1300), the term modus was used to describe a part of the overall metric organisation of a piece, comparable to a modern time signature. It referred to the division of the longa note into either three (modus perfectus) or two (modus imperfectus) breves. Similar divisions on subsequently lower levels were described by the terms tempus (referring to the division of breves into two or three semibreves) and prolatio (the division of semibreves into two or three minims).