The goal with topic maps is to achieve a one-to-one relationship between topics and the subjects that they represent, in order to ensure that all knowledge about a particular subject can be accessed via a single topic. However, sometimes the same subject is represented by more than one topic, especially when two topic maps are being merged. In such a situation it is necessary to have some way of establishing the identity between seemingly disparate topics. For example, if reference works publishers from Norway, France and Germany were to merge their topic maps, there would be a need to be able to assert that the topics “Italia”, “l'Italie” and “Italien” all refer to the same subject. The concept that enables this is that of subject identity. When the subject is an addressable information resource (an “addressable subject”), its identity may be established directly through its address. However most subjects, such as Puccini, Italy, or the concept of opera, are not directly addressable. This problem is solved through the use of subject indicators (originally called “subject descriptors” in ISO 13250). A subject indicator is “a resource that is intended ... to provide a positive, unambiguous indication of the identity of a subject.” Because it is a resource, a subject indicator has an address (usually a URI) that can be used as a “subject identifier”.