Part I - Organum
The earliest known examples of polyphonic compositions were based on existing plainchant. Organum is the addition of one or two more voices to the original chant line. These added voices run in parallel motion to the original voice. There are several manifestations of organum. The first is parallel organum, in that the added voice sings the same line as the original voice at the interval of a fourth or fifth above or below.
Notice how the added line (the lower voice) stays away from the principal line at the interval of a fifth throughout the passage. This is called parallel organum.
Here is an example of our part organum. Notice how the voices all sing in parallel motion to the principal line:
Part II - Innovations in organum
More elaborate organum eventually became more popular once people discovered that the second voice did not have to imitate the principal voice exactly. This is an example of four part organum where the added voices do not imitate the principal voice exactly.
After the invent of organum, composers began to mix the two types of composition (plainchant and organum).
The Clausula is an excerpt of organum at the end of a section of plainchant. These were usually placed on the end of a text to give certain text more importance.
Part III - Innovations in notation
By the middle of the 13th century, the notation systems needed to take into account note durations as well as pitches. Thus several new notation styles were devised. The first we will discuss is Franconian Notation. This notation style devised the Long (equivalent to a dotted half note), the Breve (equivalent to a quarter note) and the semibreve (equivalent to an eighth note) Among others:
