Aristotle's discussion of relatives in the Categories presented its eleventh- and twelfth-century readers with many puzzles. Their attempt to solve these puzzles and to develop a coherent account of the category led around the beginning of the twelfth century to the invention of relations as items which stand to relatives as qualities stand to qualified substances. In this paper, I first discuss the details of Aristotle's accounts of relatives and the related category of ‘situation’ and Boethius' commentary on them. I (...) then examine some of the earliest mediaeval commentaries on the Categories showing how the notion of relation, and in particular of individual relations, was developed. I conclude by showing how Peter Abaelard's treatment of relations in his Dialectica was part of an ongoing and sophisticated debate over the nature of relations. (shrink)