stylistic:style_transfer
Ars antiqua

The flowering of the Notre Dame school of polyphony from around 1150 to 1250 corresponded to the equally impressive achievements in Gothic architecture: indeed the centre of activity was at the cathedral of Notre Dame itself. Sometimes the music of this period is called the Parisian school, or Parisian organum, and represents the beginning of what is conventionally known as Ars antiqua. This was the period in which rhythmic notation first appeared in western music, mainly a context-based method of rhythmic notation known as the rhythmic modes.

This was also the period in which concepts of formal structure developed which were attentive to proportion, texture, and architectural effect. Composers of the period alternated florid and discant organum (more note-against-note, as opposed to the succession of many-note melismas against long-held notes found in the florid type), and created several new musical forms: clausulae, which were melismatic sections of organa extracted and fitted with new words and further musical elaboration; conductus, which were songs for one or more voices to be sung rhythmically, most likely in a procession of some sort; and tropes, which were additions of new words and sometimes new music to sections of older chant. All of these genres save one were based upon chant; that is, one of the voices, (usually three, though sometimes four) nearly always the lowest (the tenor at this point) sang a chant melody, though with freely composed note-lengths, over which the other voices sang organum. The exception to this method was the conductus, a two-voice composition that was freely composed in its entirety.

The motet, one of the most important musical forms of the high Middle Ages and Renaissance, developed initially during the Notre Dame period out of the clausula, especially the form using multiple voices as elaborated by Pérotin, who paved the way for this particularly by replacing many of his predecessor (as canon of the cathedral) Léonin 's lengthy florid clausulae with substitutes in a discant style. Gradually, there came to be entire books of these substitutes, available to be fitted in and out of the various chants. Since, in fact, there were more than can possibly have been used in context, it is probable that the clausulae came to be performed independently, either in other parts of the mass, or in private devotions. The clausula, thus practised, became the motet when troped with non-liturgical words, and this further developed into a form of great elaboration, sophistication and subtlety in the fourteenth century, the period of Ars nova. Surviving manuscripts from this era include the Montpellier Codex, Bamberg Codex, and Las Huelgas Codex.

Composers of this time include Léonin, Pérotin, W. de Wycombe, Adam de St. Victor, and Petrus de Cruce (Pierre de la Croix). Petrus is credited with the innovation of writing more than three semibreves to fit the length of a breve. Coming before the innovation of imperfect tempus, this practice inaugurated the era of what are now called "Petronian" motets. These late 13th-century works are in three to four parts and have multiple texts sung simultaneously. Originally, the tenor line (from the Latin tenere, "to hold") held a preexisting liturgical chant line in the original Latin, while the text of the one, two, or even three voices above, called the voces organales, provided commentary on the liturgical subject either in Latin or in the vernacular French. The rhythmic values of the voces organales decreased as the parts multiplied, with the duplum (the part above the tenor) having smaller rhythmic values than the tenor, the triplum (the line above the duplum) having smaller rhythmic values than the duplum, and so on. As time went by, the texts of the voces organales became increasingly secular in nature and had less and less overt connection to the liturgical text in the tenor line.

The increasing rhythmic complexity seen in Petronian motets would be a fundamental characteristic of the 14th century, though music in France, Italy, and England would take quite different paths during that time.