Crosswalks allow systems to effectively convert data from one metadata standard to another. They enable heterogeneous collections to be searched simultaneously with a single query as if they were a single database (semantic interoperability). In recent years, major efforts in metadata mapping have produced a substantial number of crosswalks. Almost all schemas have created crosswalks to popular schemas such as DC, MARC, LOM, etc. Metadata specifications may also include crosswalks to a previous version of a schema as well as to other metadata schemas. An example is the VRA Core 3.0, which lists mapped elements in target schemas VRA 2.0 (an earlier version), CDWA, and DC.