This paper will use a corpus to explore vague categorisation (e.g. prostitutes, sailors and the like) in a specific context where the participants are strangers, but where they share the same socio-cultural reference points and so can assume a critical level of shared socio-cultural knowledge when they use vague language. Unlike most work on vague language, this study looks at vague items which are not necessarily pre-textual or prototypical, but which emerge from shared knowledge. The data comprises 55,000 words of calls to an Irish radio phone-in show. Vague category markers are isolated and described in terms of form and domain of reference. It is argued that the shared knowledge required in order to construct vague categories has a common core of socio-culturally ratified 'understandings' and that the range of domains of reference of these categories is relative to the depth of shared knowledge of the participants and relative to their social relationship.