A large proportion of the music developed in Europe during the medieval period was vocal, both of a religious and secular nature. In church music, this took the form of Gregorian and other types of chants, while non-religious music consisted largely of the songs of traveling minstrels and troubadours.
(1200 - 1450)
Vocal music was, until the 9th century, written for one voice part only. Then a second, lower part was introduced, which duplicated the top melody exactly by an interval of a fifth or fourth. A third voice was sometimes added, sounding an octave below. The idea of contrary motion slowly developed, in which the lower part moved in the opposite direction to the top. While the idea of two or more voices, or polyphony, began to influence church music, secular songs continued to be written for one voice, accompanied by various instruments.
During the 12th century, vocal music became more rhythmically interesting as added parts began to include more notes than the principal melody, now called the cantus firmus.
contributed by Gifford, Katya