when electronic material is no longer accessible for six months or longer. One example of a trigger event is the case when access to the information made available is lost due to a natural event. Related Initiatives Several initiatives have developed methods in which to retain electronic documents and retain perpetual access. The most notable of these are the LOCKSS program, the CLOCKSS (Controlled LOCKSS), and the Ithaka Portico program. See also Digital preservation Software license References External links CLOCKSS official site Definition of Perpetual Access from Oxford University Press Definition of Perpetual Access from Portico DSpace official site Electronic Resource Management: Report of the DLF ERM Initiative Keepers Registry official site List of participating publishers and titles in LOCKSS LOCKSS official site Perpetual Access and Post-Cancellation Access from LOCKSS Perpetual Access to Electronic Journals: A Survey of One Academic Research Library's Licenses Portico official site Digital rights management