When authors publish their interpretations of the ideas, opinions, claims or rebuttals in the literature, they are drawing on a repertoire of well understood moves, contributing to an extended discourse. Readers also bring their own perspective to documents, interpreting them in the light of their own research interests, and initiating, for instance, new connections that may not have been intended by authors. Collaborative, social, tagging holds promise as an approach to mediating these processes via the Web, but may lack the discourse dimension that is fundamental to the articulation of interpretations. We therefore take a hybrid semiformal approach to add structure to freeform folksonomies. Our experience demonstrates that this particular brand of tagging requires tools designed specifically for this sensemaking task by providing enough support to initiate the annotation, while not overwhelming users with suggestions. We describe a tool called ClaimSpotter that aims at supporting this tradeoff, through a novel combination of system-initiated tag recommendations, Web interface design, and an expanded conception of how tags can be both expressed, and semantically linked. We then report a detailed study which analysed the tool’s usability and the tag structures created, contributing to our understanding of the implications of adding structure to collaborative tagging.