Jean-François Nieus is particularly interested in documentary productions associated with the exercise of princely power and seigneurial management, within an area stretching from the Anglo-Norman world to the Southern Netherlands. Princely and noble archives are essential for understanding the relationships of domination in the so-called "feudal" age, that of territorial principalities and seigneurial lordship, but also issues of family identity and lineage, which were central concerns of the aristocracy. "After the mid-12th century, most noble families began to keep archives, initially consisting of a few received charters, but soon enriched with their own administrative productions. Although the majority of these secular collections have now disappeared, there is evidence of their existence," he explains. The vicissitudes of the history of the great families and the French Revolution contributed to the loss of these fragile documents, so that today only a handful of archives from the 12th and 13th centuries remain.
Those studied by Jean-François Nieus nevertheless cover a wide range of types: they include "chartriers" (collections of original charters), "cartulaires" (collections of copies of charters), "formulaires " (collections of model charters and letters), "censiers" (descriptions of the property and income belonging to a seigneury), lists of vassals and fiefs, accounts, etc.
Jean-François Nieus also carries out critical editing work. He will soon publish the archives of the Béthune family (now Pas-de-Calais), as well as those of a small abbey linked to these lords, Saint-Jean-Baptiste de Chocques, whose collection, destroyed during the French Revolution, he is reconstructing.
This patient and meticulous work of discovery, deciphering, studying, and publishing sources that are sometimes very scattered helps to restore the memory of an era and enrich the documentation available to researchers.