A persistent uniform resource locator (PURL) is a uniform resource locator (URL) (i.e., location-based uniform resource identifier or URI) that is used to redirect to the location of the requested web resource. PURLs redirect HTTP clients using HTTP status codes. Originally, PURLs were recognizable for being hosted at purl.org or other hostnames containing purl. Early on many of those other hosts used descendants of the original OCLC PURL system software. Eventually, however, the PURL concept came to be generic and was used to designate any redirection service (named PURL resolver) that: has a "root URL" as the resolver reference (e.g. http://myPurlResolver.example); provides means, to its user-community, to include new names in the root URL (e.g. http://myPurlResolver.example/name22); provides means to associate each name with its URL (to be redirected), and to update this redirection-URL; ensure the persistence (e.g. by contract) of the root URL and the PURL resolver services. PURLs are used to curate the URL resolution process, thus solving the problem of transitory URIs in location-based URI schemes like HTTP. Technically the string resolution on PURL is like SEF URL resolution. The remainder of this article is about the OCLC's PURL system, proposed and implemented by OCLC (the Online Computer Library Center). History The PURL concept was developed by Stuart Weibel and Erik Jul at OCLC in 1995. The PURL system was implemented using a forked pre-1.0 release of Apache HTTP Server. The software was modernized and extended in 2007 by Zepheira under contract to OCLC and the official website moved to http://purlz.org (the 'Z' came from the Zepheira name and was used to differentiate the PURL open-source software site from the PURL resolver operated by OCLC). PURL version numbers may be considered confusing. OCLC released versions 1 and 2 of the Apache-based source tree, initially in 1999 under the OCLC Research Public License 1.0 License and later under the OCLC Research Public License 2.0 License (http://opensource.org/licenses/oclc2). Zepheira released PURLz 1.0 in 2007 under the Apache License, Version 2.0. PURLz 2.0 was released in Beta testing in 2010 but the release was never finalized. The Callimachus Project implemented PURLs as of its 1.0 release in 2012. The oldest PURL HTTP resolver was operated by OCLC from 1995 to September 2016 and was reached as purl.oclc.org as well as purl.org, purl.net, and purl.com. Other notable PURL resolvers include the US Government Printing Office (http://purl.fdlp.gov), which is operated for the Federal Depository