about information silos". They were concerned that library users had to hunt for various types of resources with various tools, an obstacle to users, resulting in underused resources. Librarians sought multidatabase search products that would collapse the silos. Emergence of the discovery layer These two features, search-engine-like interfaces and multidatabase search, began to appear in the same software systems. One author dates the uniting of these features to 2009; this would be the invention of the discovery system. Particularly if decoupled from an ILS, this united product can also be called a discovery layer. "The discovery layer still uses the information and indexing in the integrated library system (ILS), but it also searches across proprietary databases and other electronic resources, all with the goal of revealing everything that a library owns or has licensed on a given topic be it a print monograph, an electronic journal article, streaming video, or a collection of archival documents" The discovery layer can be looked on as the replacement for the OPAC. Some libraries maintain both a catalog interface OPAC and a discovery layer interface. Typical features of a discovery system Large search space: A search can search the data from different data sources (the discovery system has a comprehensive central subject index). For example, you can search a journal article or a textbook directly in the discovery system and you do not have to change from a subject database to the library catalog. Intuitive usability, like a search engine. The search is basically only a simple form, an advanced search function is not always provided. Ranking of the results according to relevance: The "best" hit is displayed first, not necessarily the newest one. A good ranking is important because many hits are often found due to the large search space. Search refinement with drill-down menus (facets): For example, a search can be restricted to all matches available online. Correction of input errors via a "Did you mean ...?" function. Autocomplete: After input to the search field, a drop-down list of suggestions appears. Exploratory search: One finds results of interest that were not specifically requested. For example, links to similar hits, entries in subject databases, or Wikipedia articles are displayed (integration of other web technologies). Examples of discovery system products Some discovery layer or discovery service products are modules of a particular ILS or database product, and are sold by that product's vendor: an