The reign of Louis VII (1137–1180) witnessed a period of cultural innovation, in which appeared the Notre Dame school of musical composition, and the contributions of Léonin, who prepared two-part choral settings (organa) for all the major liturgical festivals.
This period in musical history has been described as a paradigm shift of lasting consequence in musical notation and rhythmic composition, with the development of the organum, clausula, conductus and motet.
The innovative nature of the Notre Dame style stands in contrast to its predecessor, that of the Abbey of St Martial, Limoges, replacing the monodic (monophonic) Gregorian chant with polyphony (more than one voice singing at a time).
This was the beginning of polyphonic European church music.
 Organum at its roots involves simple doubling (organum duplum or organum purum) of a chant at intervals of a fourth or fifth, above or below.
This school also marked a transition from music that was essentially performance to a less ephemeral entity that was committed to parchment, preserved and transmitted to history.
It is also the beginning of the idea of composers and compositions, the introduction of more than two voices and the treatment of vernacular texts.
For the first time, rhythm became as important as pitch, to the extent that the music of this era came to be known as musica mensurabilis (music that can be measured).
These developments and the notation that evolved laid the foundations of musical practice for centuries.
The surviving manuscripts from the thirteenth century together with the contemporaneous treatises on musical theory constitute the musical era of ars antiqua.
The Notre Dame repertory spread throughout Europe.
In Paris polyphony was being performed in the late 1190s but later sources imply that some of the compositions date back as far as the 1160s.
Although often linked to the construction of the cathedral itself, construction commenced in 1163 and the altar consecrated in 1182.
However there was evidence of musical creativity there from the early twelfth century.

Léonin's work was distinguished by two distinctive organum styles, purum and discantus.
This early polyphonic organa was still firmly based on Gregorian chant, to which a second voice was added.
The chant was called the tenor (cantus firmus or vox principalis), which literally “holds” (Latin: tenere) the melody.
The tenor is based on an existing plainsong melody from the liturgical repertoire (such as the Alleluia, Verse or Gradual, from the Mass, or a Responsory or Benedicamus from the Office).
This quotation of plainchant melody is a defining characteristic of thirteenth century musical genres.
In organum purum the tenor part was drawn out into long pedal points, while the upper part or duplum contrasted with it in a much freer rhythm, consisting of melisms (melismatic or several notes per syllable, compared to syllabic, a single note per syllable).
In the second, discantus, style, the tenor was allowed to be melismatic, and the notes were quicker and more regular with the upper part becoming equally rhythmic.
These more rhythmic sections were known as clausulae (puncta).
Another innovation was the standardization of note forms, and Léonin's new square notes were quickly adopted.
Although he developed the discantus style, Léonin's strength was as a writer of organum purum.
The singing of organa fell into disuse by the mid thirteenth century.
Associated with the Notre Dame school, was Johannes de Garlandia, whose De mensurabili provided a theoretical basis, for Notre Dame polyphony is essentially musica mensurabilis, music that is measured in time.
 In his treatise, he defines three forms of polyphony, organum in speciali, copula, and discant, which are defined by the relationship of the voices to each other and by the rhythmic flow of each voice.
The reign of Louis VII (1137–1180) witnessed a period of cultural innovation, in which appeared the Notre Dame school of musical composition, and the contributions of Léonin, who prepared two-part choral settings (organa) for all the major liturgical festivals. This period in musical history has been described as a paradigm shift of lasting consequence in musical notation and rhythmic composition, with the development of the organum, clausula, conductus and motet. The innovative nature of the Notre Dame style stands in contrast to its predecessor, that of the Abbey of St Martial, Limoges, replacing the monodic (monophonic) Gregorian chant with polyphony (more than one voice singing at a time). This was the beginning of polyphonic European church music.  Organum at its roots involves simple doubling (organum duplum or organum purum) of a chant at intervals of a fourth or fifth, above or below. This school also marked a transition from music that was essentially performance to a less ephemeral entity that was committed to parchment, preserved and transmitted to history. It is also the beginning of the idea of composers and compositions, the introduction of more than two voices and the treatment of vernacular texts. For the first time, rhythm became as important as pitch, to the extent that the music of this era came to be known as musica mensurabilis (music that can be measured). These developments and the notation that evolved laid the foundations of musical practice for centuries. The surviving manuscripts from the thirteenth century together with the contemporaneous treatises on musical theory constitute the musical era of ars antiqua. The Notre Dame repertory spread throughout Europe. In Paris polyphony was being performed in the late 1190s but later sources imply that some of the compositions date back as far as the 1160s. Although often linked to the construction of the cathedral itself, construction commenced in 1163 and the altar consecrated in 1182. However there was evidence of musical creativity there from the early twelfth century. 
Léonin's work was distinguished by two distinctive organum styles, purum and discantus. This early polyphonic organa was still firmly based on Gregorian chant, to which a second voice was added. The chant was called the tenor (cantus firmus or vox principalis), which literally “holds” (Latin: tenere) the melody. The tenor is based on an existing plainsong melody from the liturgical repertoire (such as the Alleluia, Verse or Gradual, from the Mass, or a Responsory or Benedicamus from the Office). This quotation of plainchant melody is a defining characteristic of thirteenth century musical genres. In organum purum the tenor part was drawn out into long pedal points, while the upper part or duplum contrasted with it in a much freer rhythm, consisting of melisms (melismatic or several notes per syllable, compared to syllabic, a single note per syllable). In the second, discantus, style, the tenor was allowed to be melismatic, and the notes were quicker and more regular with the upper part becoming equally rhythmic. These more rhythmic sections were known as clausulae (puncta). Another innovation was the standardization of note forms, and Léonin's new square notes were quickly adopted. Although he developed the discantus style, Léonin's strength was as a writer of organum purum. The singing of organa fell into disuse by the mid thirteenth century. Associated with the Notre Dame school, was Johannes de Garlandia, whose De mensurabili provided a theoretical basis, for Notre Dame polyphony is essentially musica mensurabilis, music that is measured in time.  In his treatise, he defines three forms of polyphony, organum in speciali, copula, and discant, which are defined by the relationship of the voices to each other and by the rhythmic flow of each voice.
Pérotin