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What a DOI is, and how to cite one
A DOI is the most reliable way to point at a piece of research, and the quickest way to build a reference from it. What one is, where to find it, and how each style writes it.
What a DOI is
A permanent name for one work
A DOI, short for digital object identifier, identifies a published object: most often a journal article, but also books, chapters, datasets and reports. Unlike a web address, it is meant to keep working when a publisher moves or rebuilds its site.
Prefix, slash, suffix
Every DOI begins with 10., then digits that identify whoever registered it, a forward slash, and a suffix they chose. In 10.1038/nphys1170 the part before the slash belongs to the registrant and the part after picks out one paper.
It resolves through doi.org
Put https://doi.org/ in front of any DOI and the link opens the current home of the work. The DOI stays the same; only the address behind it is updated when the work moves.
Where to find one
On the article itself
Publishers print the DOI on the first page of an article, usually in the header or footer, and on the article's web page near the title or the citation details. Crossref, one of the agencies that registers DOIs, makes displaying it on that page a condition of membership.
In a database record
Library databases and indexes such as PubMed usually list the DOI with the journal details, even when the copy they link to is hosted somewhere else.
Books often have one too
Many academic books and chapters carry a DOI as well as an ISBN. Where both exist, the DOI points at the exact work or chapter, while an ISBN identifies one edition and format of a book.
How each style writes it
APA 7, MLA 9, Harvard and Chicago: a full link
Write the DOI as a complete https://doi.org/ address. That is also Crossref's own display guidance: a full link, not preceded by doi: or DOI:, and without the old dx. in the domain.
IEEE is the exception
IEEE writes doi: 10.1038/nphys1170 at the end of the reference rather than a link. It is the style's own convention, not an error, even though Crossref's guidance points the other way.
Old forms still work, but look old
A DOI written with http://dx.doi.org/ still resolves, and Crossref says those older links will keep working indefinitely. In a reference list they are simply out of date, so rewrite them rather than copy them.
When the lookup gets it wrong
The details come from the publisher
Looking up a DOI returns the metadata the publisher deposited when it registered the work. CiteRight reads it from Crossref in your browser and shows every field, because a deposit can carry a title in capitals, a missing issue number or an author list in an odd form.
Not every DOI is in Crossref
DOIs for datasets and some reports are registered with other agencies, so a lookup that finds nothing does not prove the DOI is wrong. Open it at https://doi.org/: if that lands on the work, the DOI is real and the details can be typed in by hand.
A DOI that goes nowhere
A DOI that resolves to nothing is the clearest sign that a reference was mistyped or invented. The reference checker resolves every DOI in a list at once and says which ones fail.
Questions
What does the 10. at the start mean?
Every DOI begins with 10. It marks the string as a DOI, and the digits that follow, up to the slash, identify who registered it.
Does a DOI ever change?
It is not meant to. That is the point of it: the DOI stays fixed while the address it points to is kept up to date, so a reference written today should still lead to the work later.
Why does a DOI lookup find nothing?
Either the DOI is mistyped, or it was registered with an agency other than Crossref, which is common for datasets. Try it at https://doi.org/ first; if it opens the work, enter the details by hand.
Is a DOI the same as a URL?
No. A URL is an address, and addresses change. A DOI is a name for the work, and https://doi.org/ turns it into an address that is kept current.
Build the references themselves
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