In this paper, we re-examine the notion of non-restrictive relative clauses (NRRCs) in the light of spoken corpus evidence, based on an analysis of 692 occurrences of non-restrictive which-clauses in British and American spoken English data. After reviewing traditional conceptions of NRRCs and some recent work on the broader notion of subordination in spoken grammar, we discuss the problem of identifying such clauses. We then present examples from our corpora and a concordance-based analysis of co-textual and contextual factors. A smaller sample of 214 occurrences is then subjected to a more detailed analysis, using a relational database. We conclude that non-restrictive which-clauses fall into three broad functional types: expansion, evaluation, and affirmation, with the category of evaluation being the most frequent. In all three cases, pragmatic factors, rather than semantic requirements, explicate their use. Our results also show regular collocations with copular verb-forms and with certain types of discourse markers and modal items. Turn-taking is also considered, and three recurring phenomena are discussed: which-clauses as turn extensions, the occurrence of the NRRC following a response token from the listener, and second-speaker construction of the NRRC. We use the results of our analysis to contribute to current debates on the notion of categories such as subordination in spoken grammar, on grammar and situated interaction, and on the status of probabilities in grammatical description.