Feudalism is a grouping of legal and military customs, prevalent in medieval Europe, which flourished between the 9th and 15th centuries, or any similar grouping of legal and military customs. Simply defined, it was a system for structuring society around relationships derived from the holding of land in exchange for service or labour.Although derived from the Latin word feodum or feudum (fief), then in use, the term feudalism and the system it describes were not conceived of as a formal political system by the people living in the Middle Ages. In its classic definition, by François-Louis Ganshof (1944), feudalism describes a set of reciprocal legal and military obligations among the warrior nobility, revolving around the three key concepts of lords, vassals and fiefs.A broader definition of feudalism, as described by Marc Bloch (1939), includes not only the obligations of the warrior nobility but those of all three estates of the realm: the nobility, the clergy, and the peasantry bound by manorialism; this is sometimes referred to as a "feudal society". Since the publication of Elizabeth A. R. Brown's "The Tyranny of a Construct" (1974) and Susan Reynolds's Fiefs and Vassals (1994), there has been ongoing inconclusive discussion among medieval historians as to whether feudalism is a useful construct for understanding medieval society. Abels notes that, "Western Civilization and World Civilization textbooks now shy away from the term 'feudalism'."