This is the first official musical period in Western classical music. Much of the music during this period was vocal, partly due to the religious influence and partly due to the fact that many instruments had not been invented yet. The most common musical form was the Gregorian Chant. At the beginning of this period, there was no written music however around the 9th century, some form of musical notation began to evolve. Rhythmic notation, on the other hand, did not appear until the 13th century. Famous composers from this period include Hildegard von Bingen, Guido of Arezzo and Guillaume de Machaut.