This article presents two meanings of family, one from the Barbarian epoch – in the period of Prussian paganism, the second after their conquest by the Teutonic Order, in the feudal era. It was assumed that the Prussians, like other barbarian peoples, recognized kinship along both lines (patrilineal and matrilineal), measured as “knees” (from the head through the shoulders down to the elbows and nails). This meant that the family was not a rigid and stable group, but varied in every generation. Even close relatives had different kinship circles. Only brothers and sisters belonged to the same group. It also meant that the family did not have a single seat, as well as a single authority. The first and foremost task of the family was divination, and as a consequence this also involved the reception of or payment for property. The second and equally important task of the family seems to be have been the protection of ownership. Another task of the cognitive family was a common oath. Each member of the community was obliged to stand with his relatives and assist him with an oath, but on the other hand, he could rely on consistent support from other relatives. The family in the feudal era was something else. It was built on an agnatic basis, and therefore consisted of a closed group deriving from the same ancestor down the patrilineal line. Its functions had also undergone some restrictions. The circle of people entitled to individual tasks also changed. The trend was a narrowing of this circle. Due to the fact that the cognitive family did not allow ambitious families to increase their significance and wealth, especially in multiplying their ownership of land, at the end of the pagan era in Prussia, the importance of kinship was growing on an agnatic basis, as has been the case in feudal Europe for some time. The breakdown of the old ancestral structures was finally made by the Teutonic Order. The cognitive family was only indispensable and possible in a collective society, the new era was defined by individualism and family relationships. With the rise of the new authority, the solidarity of the family disappeared. In its place an indivisible agnatic unit appeared. The disintegration of the cognitive family did not happen immediately, nor was there any single decision or legal measure from the new rulers in this matter. It was a certain process, and some of the old elements were retained into modern times. Such elements included: blood revenge and ‘weregild’ as an act of humility, the cohabitation of relatives, the importance of the matrilineal line, especially uncle–nephew, commonly held property possessed by a group of relatives, and the transfer of names down both lines. After the disintegration of the cognitive family, the circle of relatives, which was based on canon law, changed and constantly decreased. New rules of inheritance also appeared, and finally an important and new form of identifying families of the agnatic feudal era, namely heraldic emblems and seals. As a result of these changes in the 15th century in Prussia, traces of the former cognitive family are difficult to find.