This kind of system is useful, but only goes so far. However, the topic of European History can be subdivided in many ways, for instance, by nation, by economic changes, by political movements, by cultural characteristics, and so on. And of course, all of these topics are inter-related. It is necessary in the library to place books under one subject heading most of the time, because books are physical objects and can appear in only one place at a time. The computer gives us the extra power of allowing us to group books and other documents in many different ways.
Text consists of abstract discussions of ideas and their inter-relationships.
The user can also look at the titles of the documents in each group. The documents can in the cluster can have other representations as well, such as summaries, or TileBars . This re-grouping process tends to change the kinds of themes of the clusters, because the documents in a subcollection discuss a different set of topics than all the documents in the larger collection.