Abstract.  This paper discusses morphological and syntactic aspects of relative clauses in two related Southern Bantu language groups. In Sotho-Tswana, object relative clauses are formed by means of clause-initial relative complementisers which agree with the head noun. In contrast, object relatives in the Nguni languages are formed by means of relative concords which are attached to the relative clause predicate and express agreement with the subject. I suggest that the Nguni relative concords are the result of a grammaticalisation process in which early Nguni relative complementisers first turned into clitics and then into relative concords. On the basis of a detailed analysis of this process I further argue that the syntactic difference between Sotho-Tswana and Nguni relative clauses is correlated with a morphological difference between nouns in these languages.