The contracts of many database vendors grant automatic copyright permission to subscribers to establish links to articles within their databases. These links may persist over a long period of time. They are called by various names--persistent links, durable links, persistent URLs or PURLs.

Persistent links will break if the vendor deletes the document or changes the URL it goes to.

When vendors do not permit persistent links, a document's URL changes constantly, so that it is not possible to create a URL that works for more than a few minutes or hours.