occupied by a single tribe. The family unit of a tribe is called the rodzina, while a collection of tribes is a plemię. Mieszko I of Poland (c. 935 – 25 May 992) established an elite knightly retinue from within his army, which he depended upon for success in uniting the Lekhitic tribes and preserving the unity of his state. Documented proof exists of Mieszko I's successors utilizing such a retinue, as well. Another group of knights were granted land in allodium, not feudal tenure, by the prince, allowing them the economic ability to serve the prince militarily. A Polish warrior belonging to the military caste living at the time prior to the 15th century was referred to as a "rycerz", very roughly equivalent to the English "knight," the critical difference being the status of "rycerz" was almost strictly hereditary; the group of all such warriors was known as the "rycerstwo". Representing the wealthier families of Poland and itinerant knights from abroad seeking their fortunes, this other group of rycerstwo, which became the szlachta ("szlachta" becomes the proper term for Polish aristocracy beginning about the 15th century), gradually formed apart from Mieszko I's and his successors' elite retinues. This rycerstwo/aristocracy secured more rights granting them favored status. They were absolved from particular burdens and obligations under ducal law, resulting in the belief only rycerstwo (those combining military prowess with high/aristocratic birth) could serve as officials in state administration. Select rycerstwo were distinguished above the other rycerstwo, because they descended from past tribal dynasties, or because early Piasts' endowments made them select beneficiaries. These rycerstwo of great wealth were called możni (Magnates). They had the same political status and status in law as the rycerstwo from which they all originated and to which they would return were their wealth lost. The Period of Division from, A.D., 1138 – A.D., 1314, which included nearly 200 years of fragmentation and which stemmed from Bolesław III's division of Poland among his sons, was the genesis of the political structure where the great landowning szlachta (możni/Magnates, both ecclesiastical and lay), whose land was in allodium, not feudal tenure, were economically elevated above the rycerstwo they originated from. The prior political structure was one of Polish tribes united into the historic Polish nation under a state ruled by the Piast dynasty, this dynasty appearing circa 850 A.D. Some możni (Magnates) descending from past tribal dynasties regarded