Modus (medieval music)  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. 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). In the medieval theory of rhythmic organisation, a "mode" was understood as a patterned sequence of long and short values. The expressions "rhythmic mode" and "modal rhythm", however, are modern names applied to the medieval concept. Just what relationship may have existed with a metric foot in ancient or medieval poetry or poetic theory is not entirely clear . Rhythmic modes were first used by the Notre Dame School according to a classification numbered from 1 to 6. The patterns are all ternary, and vary in number (depending on the theorists' preferences) from four to nine . The six most often described, forming the nucleus of the system, are (; ):  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rhythms and thereby make the notation of complex polyphonic music possible, which was devised around 1200 AD and later superseded by the more complex mensural notation.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—a single written symbol representing two or more notes. A three-note ligature followed by a succession of duple ligatures indicated mode 1; the reverse—a succession of duple ligatures ending with a ternary on—indicated mode 2; a single note followed by a series of ternary ligatures mean mode 3 and the reverse mode 4; uniform ternary ligatures signified mode 5, and a four-note ligature followed by a chain of ternary ligatures meant indicated mode 6 .