Structuring Knowledge for Reference Generation: A Clustering Algorithm 
 
 This paper discusses two problems that arise in the Generation of Referring Expressions : (a) numeric-valued attributes , such as size or location; (b) perspective-taking in reference . Both problems, it is argued, can be resolved if some structure is imposed on the available knowledge prior to content determination . We describe a clustering algorithm which is sufficiently general to be applied to these diverse problems, discuss its application, and evaluate its performance. 