Focusing Search in Hierarchical Structures with Directory Sets. Guy Jacobson, Balachander Krishnamurthy, Divesh Srivastava and Dan Suciu. Keyword-based searches on the World Wide Web are often of limited use, because they return too many uninteresting matches. We propose here a novel mechanism that permits the user to specify directory sets to restrict the space of documents searched, and, at the same time, increase the speed of the search. We view the Web as a single, huge hierarchy, reflected in the structure of the URLs. We refer to each sub-tree in this hierarchy as a directory, and group semantically related documents from multiple directories into a directory set. Starting from a collection of pre-defined directory sets, a user can dynamically generate new directory sets by using operators in a directory set algebra, and focus keyword-based searches to documents that belong to a (pre-defined or dynamically generated) directory set. We design algorithms for efficiently evaluating expressions in the directory set algebra, and describe a technique for tightly integrating directory sets into keyword-based search engines. Finally, we validate our ideas using a realistic case study for a significant fragment of the AT&T intranet, and 211 pre-defined directory sets.