The stratification of a fighting band of vassals into distinct groups might roughly correlate with the new term "fief" that had started to supersede "benefice" in the 9th century. An "upper" group comprised great territorial magnates, who were strong enough to ensure the inheritance of their benefice to the heirs of their family. A "lower" group consisted of landless knights attached to a count or duke. This social settling process also received impetus in fundamental changes in the conduct of warfare. As co-ordinated cavalry superseded disorganized infantry, armies became more expensive to maintain. A vassal needed economic resources to equip the cavalry he was bound to contribute to his lord to fight his frequent wars. Such resources, in the absence of a money economy, came only from land and its associated assets, which included peasants as well as wood and water.