stylistic:tone
Rhythm

Concerning rhythm, this period had several dramatic changes in both its conception and notation. During the early medieval period there was no method to notate rhythm, and thus the rhythmical practice of this early music is subject to debate among scholars. The first kind of written rhythmic system developed during the 13th century and was based on a series of modes. This rhythmic plan was codified by the music theorist Johannes de Garlandia, author of the De Mensurabili Musica (c. 1250), the treatise which defined and most completely elucidated these rhythmic modes. In his treatise Johannes de Garlandia describes six species of mode, or six different ways in which longs and breves can be arranged. Each mode establishes a rhythmic pattern in beats (or tempora) within a common unit of three tempora (a perfectio) that is repeated again and again. Furthermore, notation without text is based on chains of ligature s (the characteristic notations by which groups of notes are bound to one another).

The rhythmic mode can generally be determined by the patterns of ligatures used. Once a rhythmic mode had been assigned to a melodic line, there was generally little deviation from that mode, although rhythmic adjustments could be indicated by changes in the expected pattern of ligatures, even to the extent of changing to another rhythmic mode. The next step forward concerning rhythm came from the German theorist Franco of Cologne. In his treatise Ars cantus mensurabilis ("The Art of Mensurable Music"), written around 1280, he describes a system of notation in which differently shaped notes have entirely different rhythmic values. This is a striking change from the earlier system of de Garlandia. Whereas before the length of the individual note could only be gathered from the mode itself, this new inverted relationship made the mode dependent upon—and determined by—the individual notes or figurae that have incontrovertible durational values, an innovation which had a massive impact on the subsequent history of European music. Most of the surviving notated music of the 13th century uses the rhythmic modes as defined by Garlandia. The step in the evolution of rhythm came after the turn of the 13th century with the development of the Ars Nova style.

The theorist who is most well recognized in regard to this new style is Philippe de Vitry, famous for writing the Ars Nova ("New Art") treatise around 1320. This treatise on music gave its name to the style of this entire era. In some ways the modern system of rhythmic notation began with Vitry, who completely broke free from the older idea of the rhythmic modes. The notational predecessors of modern time meters also originate in the Ars Nova. This new style was clearly built upon the work of Franco of Cologne. In Franco's system, the relationship between a breve and a semibreves (that is, half breves) was equivalent to that between a breve and a long: and, since for him modus was always perfect (grouped in threes), the tempus or beat was also inherently perfect and therefore contained three semibreves. Sometimes the context of the mode would require a group of only two semibreves, however, these two semibreves would always be one of normal length and one of double length, thereby taking the same space of time, and thus preserving the perfect subdivision of the tempus. This ternary division held for all note values. In contrast, the Ars Nova period introduced two important changes: the first was an even smaller subdivision of notes (semibreves, could now be divided into minim), and the second was the development of "mensuration."

Mensurations could be combined in various manners to produce metrical groupings. These groupings of mensurations are the precursors of simple and compound meter. By the time of Ars Nova, the perfect division of the tempus was not the only option as duple divisions became more accepted. For Vitry the breve could be divided, for an entire composition, or section of one, into groups of two or three smaller semibreves. This way, the tempus (the term that came to denote the division of the breve) could be either "perfect" (tempus perfectum), with ternary subdivision, or "imperfect" (tempus imperfectum), with binary subdivision. In a similar fashion, the semibreve's division (termed prolation) could be divided into three minima (prolatio perfectus or major prolation) or two minima (prolatio imperfectus or minor prolation) and, at the higher level, the longs division (called modus) could be three or two breves (modus perfectus or perfect mode, or modus imperfectus or imperfect mode respectively). Vitry took this a step further by indicating the proper division of a given piece at the beginning through the use of a "mensuration sign", equivalent to our modern "time signature".

Tempus perfectum was indicated by a circle, while tempus imperfectum was denoted by a half-circle (the current symbol, used as an alternative for the time signature, is actually a holdover of this symbol, not a letter C as an abbreviation for "common time", as popularly believed). While many of these innovations are ascribed to Vitry, and somewhat present in the Ars Nova treatise, it was a contemporary—and personal acquaintance—of de Vitry, named Johannes de Muris (or Jehan des Mars) who offered the most comprehensive and systematic treatment of the new mensural innovations of the Ars Nova (for a brief explanation of the mensural notation in general, see the article Renaissance music). Many scholars, citing a lack of positive attributory evidence, now consider "Vitry's" treatise to be anonymous, but this does not diminish its importance for the history of rhythmic notation. However, this makes the first definitely identifiable scholar to accept and explain the mensural system to be de Muris, who can be said to have done for it what Garlandia did for the rhythmic modes.

For the duration of the medieval period, most music would be composed primarily in perfect tempus, with special effects created by sections of imperfect tempus; there is a great current controversy among musicologists as to whether such sections were performed with a breve of equal length or whether it changed, and if so, at what proportion. This Ars Nova style remained the primary rhythmical system until the highly syncopated works of the Ars subtilior at the end of the 14th century, characterized by extremes of notational and rhythmic complexity. This sub-genera pushed the rhythmic freedom provided by Ars Nova to its limits, with some compositions having different voices written in different mensurations simultaneously. The rhythmic complexity that was realized in this music is comparable to that in the 20th century.