OCLC assigns a unique control number (referred to as an "OCN" for "OCLC Control Number") to each new bibliographic record in the WorldCat. Numbers are assigned serially, and as of mid-2013 over a billion OCNs had been created. In September 2013, the OCLC declared these numbers to be in the public domain, removing a perceived barrier to widespread use of OCNs outside OCLC itself. The control numbers link WorldCat's records to local library system records by providing a common reference key for a record across libraries.