The term 'Medieval Music' is used to describe music written during the Middle Ages, that is to say, from the 5th century (decline and fall of the Western Roman Empire) to the early Renaissance. Almost ten centuries of music practice are contained in this definition, and there are many identifiable historical moments in what we may call medieval music: early chant music, organum polyphony, ars antiqua, ars nova, ars subtilior, etc. During these periods the foundation was laid for the notational and theoretical practices that would shape western music into what it is today. Medieval music is a difficult subject for research, given the difficulties that arise from studying an art only conserved through a notation which is vague by today's standards. The study of medieval instruments is a valuable aid in understanding the popular music practice from the period. Notable composers from the medieval period include William of Aquitaine, Leonin, Perotin, Adam de la Halle and Guillaume de Machaut.