Basic DOI Syntax
A Digital Object Identifier (DOI), is a unique string created to identify a piece of intellectual property in an online environment. Detailed information about the DOI itself can be found in the DOI Handbook at http://www.doi.org/hb.html. For the purpose of assigning DOIs and registering them with CrossRef, you need only familiarize yourself with the contents of this brief document.
The DOI is made up of two components, a prefix and a suffix, separated by a forward slash. See figure 1.
All DOI prefixes begin with "10" to distinguish the DOI from other implementations of the Handle System followed by a four-digit number or string (the prefix can be longer if necessary). In general each member has one prefix, but it is possible for members to have multiple prefixes (e.g. a prefix for each journal title). The suffix is determined by the publisher -- see the section below on creating a DOI suffix scheme. The DOI is case insensitive (e.g. 10.1006/abc is the same as 10.1006/ABC) and the suffix must be unique within a prefix.
How to get a DOI prefix.
Your unique six-digit DOI prefix will be assigned to your organization by CrossRef and is included in the CrossRef membership fee. It is not necessary to get a DOI prefix prior to joining CrossRef or to pay a separate fee to the International DOI Foundation (IDF).
It is worth noting that once assigned to a piece of intellectual content, the DOI is permanently assigned to that content regardless of the owner. While specific publishers must assign DOIs using their unique prefix, once a DOI is assigned, ownership and control of individual DOIs can be transferred. For example, if ownership of the journal "Good Science" passes from Publisher A to Publisher B, control of the DOIs for articles in "Good Science" will be given to Publisher B, who can then update the metadata and URLs for those DOIs. Publisher A will continue to assign DOIs using their existing prefix. Therefore, the prefix of a DOI does not reliably identify the publisher.
CrossRef Guidelines on DOI Suffix Creation.
In order to assign DOIs to your articles or other content items, you must come up with a DOI assignment scheme. Since the DOI prefix is provided to you, the next step is the DOI suffix.
The DOI suffix for a given journal article or content item must be created and assigned by the publisher or other organization with legal authority to register the content. The DOI suffix has a very flexible syntax. It may be any alphanumeric string, consisting either of a single node or multiple nodes. A node is a portion of a character string. A single node has no delimiters (e.g., periods, colons, pipes, etc.). An example is "123456". A character string with multiple nodes must have its nodes separated by delimiters (e.g., periods, colons, pipes, etc.). An example is "12.34.56". The DOI is case insensitive (e.g. 10.1006/abc is the same as 10.1006/ABC) and the suffix must be unique within a prefix.
Because the DOI is an opaque string intended to remain unique and persistent throughout changes in ownership and location of the content, no specific or descriptive information needs to be given in the DOI. Such information forms the metadata associated with each DOI. Bibliographic information can be used in a DOI string, but it will have no meaning within the CrossRef or DOI system. Metadata is submitted along with the DOI and URL. Existing identifiers can also be used for the DOI suffix (e.g. ISBN, PII, or an existing internal numbering scheme).
The following is a summary of the guidelines for the DOI suffix:
- The suffix should be as concise as possible, in consideration of human readability. DOIs will be displayed online and in print and will be re-typed by end users.
- The suffix should be unique within the prefix and is case insensitive.
- The suffix should reflect a consistent, logical system that can be easily documented and readily understood by employees of your organization, so that the task of assigning DOIs can be passed from one employee to the next. You might therefore want it to include existing internal identifiers in use within your organization.
It is possible to use traditional bibliographic metadata (such as journal, volume, and page) in constructing DOI suffixes since this metadata and the DOI are both persistent. However, some issues need careful consideration before choosing such an approach. DOIs should be active as soon as articles appear online. Choosing a scheme that is tied to page numbers may preclude doing this if articles are to be put online before pagination can be done for a completed print issue or if the articles are only published online.
One possible solution used by the American Physical Society is to do away with pagination altogether and use an article identifier (APS uses a six digit number) that appears in print and online in place of page number. Please refer to the sample DOIs below.
In general, a DOI suffix should not be considered "derivable." Although some DOIs may be generated according to a formula or algorithm, it is preferable to look them up in CrossRef, as there is no guarantee that a generated DOI has been registered with CrossRef or that it will resolve.
- For registering CrossRef DOIs for books and reference works please refer to the Best Practices for Books http://www.crossref.org/06members/best_practices_for_books.html.
- Consider URI Syntax (http://www.ietf.org/rfc/rfc2396.txt), and please refer to the Unicode character set (www.unicode.org). Some characters like ampersands and asterisks cause problems in URLs and need to be encoded. UTF-8, which preserves ASCII characters, is the required coding. Because SICIs include special characters, using them within a DOI can lead to difficulties in encoding. See the NISO DOI Syntax standard for more information - http://www.niso.org/standards/standard_detail.cfm?std_id=480.
- Suffix nodes may be used to reflect hierarchical information or levels of granularity. For instance, the first node might be a multiple-letter code for the journal title, while successive nodes encode year of article acceptance and order of article acceptance. This is the scheme used by Academic Press, with resulting DOIs like doi:10.1006/jmbi.1998.2354. Similarly, in the case of electronic books, the AAP recommends the following syntactic structure for the DOI suffix:
/Whole work. Next granular level. Next granular level. Etc.
- DOI suffixes should be extensible, and the suffix nodes may be used for this purpose. For instance, in the future, parts of articles such as figures, graphs, and supplementary materials might be assigned DOIs. Using the Academic Press example above, the second figure in this article might be assigned the doi:10.1006/jmbi.1998.2354.f002.
Sample DOIs. Crossref publishers have established many varying schemes for creating their DOI suffixes. Here is a list of sample DOIs for some current CrossRef publishers:
Assigning DOIs to Different Formats (Work vs. manifestation.) DOIs can be assigned to any type of intellectual property in any medium. Detailed information about DOIs can be found in the DOI Handbook http://www.doi.org/hb.html. More specific information about works vs. manifestations can be found in chapter nine of the Handbook. As a matter of current policy, the CrossRef DOI identifies the work, not its various potential manifestations. This means separate DOIs are not assigned to each format of a given article. The print, PDF, and HTML versions of the same article will all share the same DOI. Different versions of the same article can be pointed to from the response page to which the DOI resolves. As the DOI system gains sophistication and can easily handle multiple resolutions from one DOI, CrossRef's policies will change accordingly.
CrossRef as IDF Registration Agency
The IDF (International DOI Foundation) appoints Registration Agencies (RAs) to accept deposits of DOIs, URLs, and metadata for a given Community of Interest. CrossRef is an IDF RA for scholarly publishers and accepts deposits for journals, conference proceedings and books. CrossRef holds the metadata associated with DOIs and deposits the DOIs and URLs in the central DOI system. CrossRef also provides a retrieval system for the DOIs it registers.
CrossRef works with the IDF on general DOI policy and technical issues. Many CrossRef members are also members of the IDF in order to participate in general DOI developments, however, CrossRef membership is all that is necessary to deposit and retrieve DOIs. Separate IDF membership is not necessary but does provide important support for the DOI system.
CrossRef policies on DOI deposit. NOTE: It is a requirement that DOIs be deposited as soon as possible after online publication of the content. While a DOI may have been created and assigned prior to publication, it is extremely important for the reliability of the CrossRef resolution system that such pre-publication DOIs remain for internal use only, and that they are not displayed online or in print or distributed to secondary publishers prior to online availability of the content. Until a DOI and its metadata are deposited with CrossRef, the DOI will not work and the user will get an error message. CrossRef regularly receives error reports of users trying to resolve DOIs that have not been registered with CrossRef. If DOIs are not registered, they will not function.
When metadata and DOIs are deposited with CrossRef, the publisher must have active response pages in place so that they can receive incoming links. As soon as metadata and DOIs are deposited in CrossRef, other users of the system will be able to retrieve the DOIs and create links. A minimal response page consists of a full bibliographic citation displayed to the user. Additionally, this response page must display some mechanism by which the user can gain access to the full text. Access is completely controlled by the publisher. Most CrossRef publishers take users to the abstract page and permit authenticated users to access the full text automatically. If the full text is available at no charge, users can access it immediately. Many publishers also present unauthenticated users with pay-per-view options.
It is the publisher's responsibility to maintain the accuracy of the metadata, DOIs, and URLs. There is no charge for sending updates or revisions to previously submitted records.
Rules about depositing content and what DOIs resolve to can be found in the Membership Qualifications and Rules - http://www.crossref.org/08downloads/memberrules.pdf - and the PILA Membership Agreement - http://www.crossref.org/08downloads/CR_MembershipAgreement.pdf.
Please note that it is not necessary to register DOIs separately with the IDF since this will be done automatically as part of the CrossRef metadata submission process. For technical details on the metadata submission process, please read the CrossRef technical information - http://www.crossref.org/02publishers/23how_start.html
Use and Display of DOIs
Every DOI is registered with a standard web URL. At present one URL is registered for one DOI, but in future multiple URLs (and other data types) will be registered with each DOI. The core functionality of the DOI system is to resolve a DOI to the registered URL. For a detailed explanation of resolution by the DOI System and multiple resolution see the DOI Handbook: http://www.doi.org/handbook_2000/resolution.html.
A DOI should be preceded by a lowercase "doi:". For example, the DOI 10.1006/jmbi.1998.2354 would be displayed as doi:10.1006/jmbi.1998.2354. Using the lowercase DOI follows the URI specification http://www.ietf.org/rfc/rfc2396.txt where FTP and HTTP are written as "ftp:" and "http:".
The DOI Proxy server - http://dx.doi.org/ - resolves DOIs. To resolve a DOI via a standard web hyperlink, the DOI number itself should be attached to the address for the proxy server. For example, the DOI 10.1006/jmbi.1998.2354 would be made an active link as http://dx.doi.org/10.1006/jmbi.1998.2354. It is important to note that in this example of a URL, the DOI number 10.1006/jmbi.1998.2354 incorporated in a URL and transported by the HTTP protocol and therefore has to follow guidelines for URIs. The syntax for URIs (a URL is a URI - http://www.ietf.org/rfc/rfc2396.txt) is much more restrictive than the syntax for DOIs and some characters are reserved and will need encoding. For information on the DOI Syntax and character issues see the NISO DOI Syntax standard - http://www.niso.org/standards/standard_detail.cfm?std_id=480.
In the future HTTP may no longer be the dominant protocol and browsers like Internet Explorer will handle DOIs natively. This means that doi:10.1006/jmbi.1998.2354 would be picked up by the browser and automatically resolved without the addition of the http://dx.doi.org/. For more information on this, see section 6.5 of the DOI Handbook - "The resolution interface with Handle System technology" - http://www.doi.org/handbook_2000/resolution.html#6.5
However, at the present time a DOI should be displayed as doi:10.1006/jmbi.1998.2354 and the DOI identifier itself can be an active link as in doi:10.1006/jmbi.1998.2354.
Display of DOIs
A main purpose of DOIs in the CrossRef process is to create persistent links to full text content, especially in reference links. DOIs for references are retrieved from CrossRef and added to references lists. How the links are actually added depends on the publisher's online system. However, DOIs should be considered a part of the basic bibliographic information for an article.
A DOI for a piece of content (article, book chapter, conference proceeding) should also be displayed in the header information for the online AND print versions of the content (see Figure 2). The DOI can also be used for citing the content. For example, many publishers instruct readers to use a DOI to cite an online article published without a volume, issue or page number.
Figure 2 - DOI in Bibliographic Header
Displaying DOIs in Print:
A DOI assigned to the electronic version of an article should also be included in the header for the print version of that article. To make the DOI system transparent for readers, we recommend listing the DOI -- for example, as above in Figure 2 -- as "doi:10.1006/jmbi.1998.2354". In order to show users how to resolve a DOI, it is possible to have the DOI followed by: "available via http://dx.doi.org/". This will take users to a form where the DOI can be entered and resolved
DOI in a citation:
DOIs do not replace traditional bibliographic citations but are a very useful addition, especially if articles are published online with volume, issue, and page numbers.
A citation of the Science article with a DOI would be:
Kornack, D. Rakic, P. (2001). Cell Proliferation Without Neurogenesis in Adult Primate Neocortex. Science. 294 (5549), 2127-2130, doi:10.1126/science.1065467.
A citation to a Nature article published in the Advanced Online Publication process without volume, issue or page number would be:
"Cell biology: A cat cloned by nuclear transplantation" Nature AOP, Published online: 14 February 2002, doi:10.1038/nature723.
DOI as a reference link:
The most common method of using the DOI in a reference link is to use "CrossRef" as the link label in brackets following the reference along with any other links for the article.
The Science article above has reference links. An example is reference 36:
Behind the display text of "CrossRef" is the URL http://www.sciencemag.org/cgi/external_ref?access_num=10.1038/35016083&link_type=DOI
This links sends the DOI for the reference through the local HighWire system that resolves outgoing links and passes them to the DOI Proxy (http://dx.doi.org/)
An example of a reference link has been taken from an article in Development Growth and Differentiation, a Blackwell Publishing journal. The citation for the source of the reference is:The most common method of using the DOI in a reference link is to use “CrossRef” as the link label in brackets following the reference along with any other links for the article.
Uchiyama, H, et al. Cloning and characterization of the T-box gene Tbx6 in Xenopus laevis. Development Growth & Differentiation 43 (6), 657-669. doi:10.1046/j.1440-169X.2001.00606.x
A reference in the article is listed as:
Ruvinsky,I., Silver,L. M., Ho,R. K.1998. Characterization of the zebrafish tbx16 gene and evolution of the vertebrate T-box family. Dev. Genes Evol. 208, 94-99.
Behind the "CrossRef" logo is the URL http://dx.doi.org/10.1007/s004270050158. This link sends the DOI for the reference to the DOI proxy server (http://dx.doi.org/). In an automatic redirect, the URL registered for the DOI is returned to the user's browser and the user is taken to the URL. In most cases the user will not even be aware of the re-direct.
In another example, the following article from Science:
Wood, Derek W, et al. The Genome of the Natural Genetic Engineer Agrobacterium tumefaciens C58. Science 2001 294:2317-2323 doi:10.1126/science.1066804
Has the following reference to Nature:
51. J. A. Downie and J. P. Young, Nature 412, 597 (2001) [CrossRef][ISI][Medline].
Behind the "CrossRef" link is the URL http://www.sciencemag.org/cgi/external_ref?access_num=10.1038/35088167&link_type=DOI
The URL is a link to a program that tracks outgoing links from Science. The program takes the doi "10.1038/35088167" and sends it to the DOI proxy server, http://dx.doi.org/ in the standard form http://dx.doi.org/10.1038/35088167.
CrossRef recommends that "CrossRef" be used as the display text for reference links. Some publishers use "Article" or make the actual journal citation the link. For example, a reference in AIP's Applied Physics Letters is:
18 M. Kiy, I. Gamboni, U. Suhner, I. Biaggio, and P. Günter, Synth. Met. 111-112, 307 (2000). [INSPEC]
Behind the text "Synth. Met. 111-112, 307 (2000)" is the URL http://dx.doi.org/10.1016/S0379-6779(99)00453-1. This is doi:10.1016/S0379-6779(99)00453-1 placed within a URL pointing to the DOI proxy server.
More information on this can be found in the "CrossRef Branding Guidelines".
Instructions for Authors
Publishers should include information about using DOIs in their instructions for authors.
For example, Nature has information for authors for its Advanced Online Publication http://www.nature.com/nature/journal/vaop/ncurrent/about.html where articles are published online before being assigned volume, issue, and page numbers.
Springer provides information to its authors on Online First http://www.springerlink.com. Articles in Online First are published before being assigned volume, issue, and page numbers, and DOIs are used to cite the articles
- DOIs, along with volume, issue and page numbers, should be part of the standard bibliographic metadata for an article
- DOIs should be displayed in bibliographic headers for online AND print articles
- In citations DOIs should be presented as doi:10.1038/35016083 (doi should be lowercase and no space should be between the doi: and the start of the DOI string)
- Wherever possible, the DOI should be an active link but the http://dx.doi.org/ need not be displayed (e.g. doi:10.1126/science.1065467)
- In a reference link to full text, CrossRef recommends that "CrossRef" be used as the display text for the link. More information on this can be found in the "CrossRef Branding Guidelines"
Information for the end-user
If you encounter a DOI in text or header information, you can resolve it by embedding it in an HTTP hyperlink to the DOI proxy, http://dx.doi.org, which redirects the DOI to the currently registered location for this content item. For example, doi:10.1006/jmbi.1998.2354 can be resolved as http://dx.doi.org/10.1006/jmbi.1998.2354. If you click on this link, you will arrive at the appropriate response page for this article at the Journal of Molecular Biology web site.
There you will see that the DOI is included in article header information or on the title page. To include the DOI in a citation to this article, you simply append it at the end, prefaced by "doi:" as follows:
Brian G. Turner, Michael F. Summers. "Structural Biology of HIV." Journal of Molecular Biology, 285(1), pp. 1-32. http://dx.doi.org/10.1006/jmbi.1998.2354.