Modus

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. Modal scales 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 in pairs (authentic/plagal), in which case they were usually named protus (first), deuterus (second), tertius (third), and tetrardus (fourth), but sometimes also named after the ancient Greek tonoi (with which, however, they are not identical). Modus (modal notation)