In the early modern period, libraries were probably the most important place of work for historians. They were used as a kind of archive, where historians could find all sorts of records, be they original documents or copies. Based on the case of the Royal Library in eighteenth-century Paris, this study aims to investigate the chain of documentary acts which gave it a para-archivistical function – which it retains to this day. First of all, I will discuss the constitution of scholars' and bureaucrats' private collections and their incorporation in the Royal Library from the final decades of the seventeenth century onwards; then the various operations of classifying, cataloguing and filing that blurred the initial rationales of the 'archive avatars' developed by previous owners; finally the uses of this peculiar material, be they documentary (by scholars or royal officials) or pragmatic (by families wishing to clarify their genealogy or private individuals involved in court cases). In the early modern period, libraries were probably the most important place of work for historians.