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* PRONOM Unique Identifiers


The PRONOM Persistent Unique Identifier (PUID) is an extensible scheme for providing persistent, unique and unambiguous identifiers for records in the PRONOM registry. Such identifiers are fundamental to the exchange and management of digital objects, by allowing human or automated user agents to unambiguously identify, and share that identification of, the representation information required to support access to an object. This is a virtue both of the inherent uniqueness of the identifier, and of its binding to a definitive description of the representation information in a registry such as PRONOM.

At present, the PUID scheme has been confined to one particular class of representation information: the format in which a digital object is encoded. Formats are considered a particular priority for such a scheme. No existing, universally applicable system provides for this. Unix ‘magic numbers' and Macintosh data-forks do provide some of this functionality, but the same is not true within DOS or Microsoft Windows environments. The three-character file extension is neither standardised nor unique, and is interpreted differently by different environments. Equally, the IANA MIME-type scheme does not provide sufficient granularity or coverage to satisfy the requirements for unique identifiers. The PUID scheme has been developed for the single purpose of providing such identifiers.

The scheme has been adopted as the recommended encoding scheme for describing file formats in the latest version of the e-Government Metadata StandardExternal link - opens in a new window. This means that, for the first time, a consistent, persistent and highly granular scheme for describing file formats will be in use across the UK government. The scheme is designed to be extensible, and will be expanded in future to include other classes of representation information in PRONOM, such as compression methods, character encoding schemes, and operating systems.

PUIDs can be expressed as Uniform Resource Identifiers (URIs) using the "info:pronom/" namespace, details of which are available from the info URI registryExternal link - opens in a new window. Neither the PUID scheme, nor its expression as an info URI, supports any inherent dereferencing mechanism, i.e. a PUID does not resolve to a Uniform Resource Locator (URL). However, The National Archives is planning to develop a range of services to expose PRONOM registry content, including a resolution service for PUIDs.

The assignment of new PUIDs is managed by The National Archives, and suggestions for new PUIDs are welcomed. To request the assignment of a new PUID, please email link. Upon receipt, The National Archives will evaluate the request, undertake any necessary research, and assign a PUID as appropriate. You will be notified of the result of the request by email, within 10 working days, although the time required to research and assign a new PUID, and to publish it via the registry, will be dependent upon the availability of the necessary technical information, and the complexity of the format.

The National Archives announces modification to x-PUID proposal:

After receiving feedback from the PRONOM user community, The National Archives has made the decision to modify their proposal to phase out x-PUIDs. PRONOM users have indicated that stability is of a higher priority than uniformity in the PUID scheme, and as an institution that is responsive to the needs of our user base, The National Archives will continue to support all existing x-PUIDs on PRONOM. In future, however, no further x-PUIDs will be created; all formats will be given a full PUID from now onwards.

The benefit of this approach is that no repositories or users of PRONOM will need to re-characterise their collections, since format identification will remain persistent.

As always, we welcome your comments on this proposal for PRONOM, and on any other matter relating to the PRONOM service. Please feel free to contact us on

A detailed description of the PUID scheme is available here:
Download as a PDF documentPDF document - link opens in a new window(78kb)
Download as a PDF documentRich text format - link opens in a new window(1,206kb)

You can search the PRONOM Technical Registry for details of all assigned PUIDs here:

Search PRONOM Unique Identifier

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