How do collections produce knowledge? Are libraries intrinsically about building knowledge? How do library holdings make sense? Creating libraries and collections, whether real or virtual, is in itself an act of building knowledge that goes beyond the simple sum of their content. The very fact of structuring knowledge changes it: the type of classification chosen creates connections and encourages interdisciplinarity or, conversely, cancels out the interchange between sources. In digital libraries, the choice of the organising principle and tools allowing documents to be connected is the key factor in deciding how the sources are to be exploited. This approach to knowledge – field by field rather than universal – is an effective way to create “rich” links between sources, thereby encouraging interdisciplinarity.