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Tracing the medieval roots of the conjugal family in the country of Holland, and especially among the ruling and social elites of the town of Leiden during the lower middle ages, the author tries to offer a modest contribution to what is sometimes called the debate on the progressive nuclearization of the family. Arguing that the importance of lineage and clan structures was limited, and that the protective and controlling role of the "cognatio" diminished during this period, this paper tries to show that several forces undermining the traditional family and favoring the long-term rise of the conjugal family can be identified in fifteenth-century Leiden. The causes of this complicated process are not only to be found in the state formation process, the penetration of Roman law, or in the particular dynamic demographic and economic developments in the country of Holland. The specific notions of hereditary and matrimonial law also encouraged the formation of the nuclear family and even affected feudal customary laws.