In the eleventh century, a major development took place. Two composers named Léonin and Pérotin combined all the styles of organum, creating works with three independently moving melodic lines (organum triplum) as well as a sustained pitch (drone organum) in the tenor. This sustained pitch is called a cantus firmus: it is actually a Gregorian chant sung very, very, very slowly.
This style of music is called Notre Dame School after Notre Dame Cathedral in Paris, where Léonin and Pérotin worked.
Keeping so many voices together required a further development in musical notation: the addition of meter. There are two aspects of meter - beat and subdivision - which must be reconciled in musical notation.
Viderunt Omnes
Sederunt Principes
The subdivision of each beat refers to how the impulse itself is divided. There are two basic types of subdivision: simple, where each beat divides into two; and compound, where each beat divides into three.
Simple Subdivision
Compound Subdivision
The beat refers to how metric impulses are grouped together. There are two basic types of beat: duple, where impulses are grouped in two; and triple, where impulses are grouped in three.
Duple Beat
Triple Beat
These two aspects of beat produce four possible meters: simple duple (ex. 2/4, 4/4), simple triple (ex. 3/4), compound duple (ex. 6/8, 12/8), and compound triple (ex. 9/8). The most common meter today is simple duple, but the most common meter during the medieval period was compound duple.
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