Much of Western Europe experienced increased prosperity and cultural revival in the 11th century, which promoted the growth of new cities, universities, roman architectural style and experimentation in musical art. The new trends in music included:

1. Polyphony, music of two or more independent voices, was becoming prominent in the church.

2. The development of precise notation allowed composers for the first time to fix a work in definitive form and transmit it accurately to other performers.

3. Written composition began to replace improvisation as a way to create new works.

4. Music was increasingly structured by principles of order, such as the eight church modes and rules governing rhythm and consonance.

The early polyphony that we call organum was probably improvised before it was written down. Motion in parallel intervals and heterophony appear in many musical cultures and were probably practiced in Europe. Polyphony was described in Musica enchiriadis and Scholica enchiriadis, a treatise and textbook from ca. 900. In this early organum, an added voice (organal voice or vox organalis) appears below a chant melody (principal voice or voix principalis), moving either in parallel motion at the interval of a fourth or fifth (parallel organum) or in a mix of parallel and oblique motion (,B>organum with oblique motion).

In free organum (or note-against-note organum) of the 11th century, the added voice usually sings above the chant (although the voices may cross), moving most often in contrary motion to the chant and forming consonant intervals with it (unison, fourth, fifth, and octave). Only those portions of the chant that were sung by soloists were set polyphonically, so that performance sections of polyphony alternate with sections of chant.

New types of polyphony, called Aquitanian polyphony, appeared early in the 12th century in southern France and Spain. In this florid organum, the chant is sustained in long notes in the lower voice (called the tenor), while the uppter voice sings from one to many notes above each note of the tenor. This style was called organum, organum duplum (double organum), or organum purum (pure organum), and organum was also used to refer to a piece that used this style. When voices move in similar measured rhythm, the texture is called discant Polyphonic settings of Latin poems called versus are the earliest polyphony not based on chant. Manuscripts for these types of polyphony use score notation (one part above the other, with notes that sound together aligned vertically), but do not indicate rhythm.

Notre Dame Organum is the label given to the polyphonic style that developed primarily in northern France from the 12th to the 14th centuries. The primary composers of this organum are Léonin (ca. 1135-ca. 1201) and Pérotin (ca. 1170-ca. 1236), who worked in Paris at the Notre Dame Cathedral. The style spread across western Europe.

Lèonin wrote or compiled the Magnus liber organi (The Great Book of Organum), a cycle of organa for the solo portions of the responsorial chants of the Mass and Office (Graduals, Alleluias, and Responsories) for the entire church year. His organa are in two voices and alternate sections of organum with sections in discant style, called clausulae (singular clausula). Discant, which is a Latin word for singing part, generally refers to a treble part, but more specifically for Medieval music refers to the improvised or written polyphony in which voices move at the same speed. The discant sections use rhythmic modes. The upper voice of the organum may be in free rhythm, although some musicians treat it in modal rhythm.

Pérotin and his contemporaries revised Léonin's work, writing discant clausulae to replace sections of organum and substitute clausula in place of older sections of discant. The tenors often repeat rhythmic patterns and segments of melody. Pérotin also wrote new organa in three and four voices, in which the upper parts are in rhythmic modes and often use voice exchange. The new style of long notes in the tenor and measured phrases in modal rhythm in the upper parts is called copula.

Rhythmic modes are patterns of long and short notes that developed during the 12th and early 13th centuries. Notation was created to indicate these patterns and by about 1250, they were codified as six rhythmic modes, each indicated by a different succession of note groupings know as ligatures. The modes were based on division of a threefold unit called perfectum.

The polyphonic conductus is a setting of a metrical Latin poem (like the earlier monophonic conductus and the versus). The tenor is newly written and is not based on chant. The two, three, or four voices move in similar rhythm and declaim the text together, in almost syllabic style. This nearly homorhythmic and syllabic texture is know as conductus style. (Note that our modern ears may hear this style as homophonic). Some other types of music were sometimes written in this style. Long melismas called caudae appeared in some conductus, particularly at the beginning or end of the piece. As in the organa and discant clausula of the Notre Dame composers, vertical consonances of the fifth and octave are prominent throughout and are required at cadences. The conductus and organum styles became less popular after 1250.

The motet (from the French word mot meaning word) was a new type of independent composition that appeared in the first half of the 13th century. Musicians produced this genre by adding new words to the upper voices of a discant clausula. The motet's duplum (voice part set against tenor that had the original melody) is called the motetus. Composers freely reworked existing clausulae and motets, adding or substituting new upper lines and texts. Motets were also newly composed, instead of being taken from existing clausulae, but still used fragment of chant in their tenors; later in the 13th century, secular tunes were also used in the tenors. Tenors were often aid out in repeating rhythmic patterns and were probably played rather than sung. Once removed from the liturgy, motets came to be sung on secular occasions. Their texts could be sacred or secular and need not relate to the text of the tenor. In three-voice motets, there were often two texts, both in Latin, both in French, or rarely, one in each language. Although the texts differ between voices, they are usually on related subjects. A motet is identified by a compound title with the first word(s) of each text, including the tenor. Frequently, the upper voices would cadence at different places to maintain the forward sense of rhythm.

Motet texts were often written to existing music and had to follow its shape. Frequently the same vowels or syllables appear in different texts, binding them together through sound as well as sense. Later motet texts are predominantly secular, the most are love songs.

The Franconian motet type, which became popular in the second half of the 13th century, had upper voices that moved more quickly and had a longer text than the middle voice, while the tenor remained the slowest voice. This motet style was named after the composer and theorist Franco of Cologne (fl. ca. 1250-1280). The Petronian motet style, named after Petrus de Cruce (fl. ca. 1270-1300) , had even more notes and shorter note values in the top voice. These later motets were harmonically similar to earlier ones, with perfect consonances on the main beats and free dissonance in between. Toward the end of the century, cadences began to be standardized, with the lowest voice moving down by step and the upper voices up by step to form a fifth and an octave above the lowest voice. This type of cadence has the outer voices expanding from a harmonic sixth to an octave, and the bottom and middle voice moving from a harmonic third to a fifth.

Hocket, from the French word for hiccup, is a technique in which a melody is interrupted by rests, while the missing notes are supplied by another voice. It was used in motets and conductus in the late 13th and early 14th centuries. Works that use it extensively, whether vocal or instrumental, are called hocket.

The notation for the rhythmic modes depended on patterns of ligatures, but the syllabic text-setting of motets made ligatures impossible. This required notation that indicated the duration of each note. Franconian notation, codified by our theorist friend Franco of Cologne in Ars cantus mensurabilis (The Art of Measurable Music, ca. 1280), solved this problem by using different note shapes for different relative values. Modern notation uses the same concept. His notation was often used in choirbook format, in which the voices all appear on the same or facing pages but they are not aligned. Some even printed lines sideways for the singers who might be off to one side of these often very large books. Notation of the 13th century used four note shapes: the double long, the long, the breve, and the semi-breve (the modern-day whole note). The basic time unit was the tempus (pl. tempora), the length of one unaltered breve, and there were three tempora in a perfection, equivalent to a modern measure of three beats. A long was either perfect (three tempora) or imperfect (two), a breve could be altered to be two tempora, and divisions of the breve were notated by semibreves. Although this notation still emphasized threefold divisions, it gave new freedom from the rhythmic modes.

In summary, polyphony developed, from the 11th through the 13th centuries, as a process of elaborating on existing works. New voices were added to existing chants in note-against-note organum, florid organum, and discant. New discant clausulae substituted for older sections of organum or discant. Words added to the upper parts of clausulae-a kind of troping--produced motets. Additional voices and texts could be added in turn, and motets were also newly composed using segments of chant and other melodies. At each step, composers elaborated on existing material. New technical developments focused on rhythm and notation, from the rhythmic modes to Franconian notation. Whereas until the early 13th century almost all polyphony was sacred, by the end of the century secular texts were also being set polyphonically.