The idea(l) of ‘legitimate peripheral participation’ remains at the heart of debates over the nature and potential of communities of practice. Yet the question of how the legitimacy or otherwise of participation is actually established is seldom addressed. In this article, we focus on ‘legitimacy’ as figure instead of ground. We attend to the ‘displays of competence’, and their associated ‘labours of division’, by means of which ‘practitioners’ claim recognition and are made recognisable to each other as members, or non-members, of an ‘us’. We seek to understand how members come to recognise particular ‘doings’ and forms of knowledge as belonging (or not belonging) to a particular practice. How is the common ‘domain’ (communis) of practice settled (or un-settled) in the course of specific performances of membership? Empirically, the article draws upon a 2-year investigation of how community of practice boundaries and participation were negotiated in ‘UltraGlass Plc’, a multinational manufacturing company, and specifically of the failure of ‘community’ to cohere around practices.