DOIs and URLs


The Chicago Manuel of Style (17th ed.) suggests that citations of online sources should include either a hyperlink (a URL), an identifying series of numbers and letters (a DOI), or some other means that allows us to find the source on the web.

Order of Priority

Wondering what type of identifier is best for online sources? Here are your options, ranked from optimal to worst case scenario:

  1. A DOI (Digital Object Identifier). E.g.,
  2. A permalink. This is a URL (Uniform Resource Locator) supplied on the web page as a link that remains stable over time. E.g.,
  3. A basic URL. E.g.,
  4. A Database title. Use only if the URL leads to a page that both requires login access and lacks inadequate bibliographical information.

Just go down the list and pick what’s available. As you do so, here are a few more things to keep in mind:

  • Try shorten very long URLs. Often you can navigate back a page or two and find a shorter form. This is especially the case if you’re doing research with Google and you find a quote from a book or article excerpt.
  • However, avoid abbreviated URLs that are meant just for social media sharing or for temporary usage. E.g., a “” link.
  • If a link ends in a slash (/), you can leave it in.
  • Don’t forget to start DOIs and URLs with http:// (or https://).
  • For more information about DOIs, please see our detailed introduction, though do note that many of the specific rules apply to the APA style guide.

URL Line Breaks

If you’re printing out a text with URLs, you’ll want to break them up to avoid awkward gaps at the end of a line. You can split a URL in the following places:

  • After a colon or double slash (//)
  • Before a single slash (/), period, comma, hyphen (-), underline (_), question mark, number sign (#), percent symbol (%), or tilde (~).
  • Before or after an equals sign (=) or ampersand (&).

Only rarely should you break up a URL between syllables.

Here is an example:


Note that you don’t have to add a hyphen to indicate where you’ve added a line break.

Digital Object Identifiers (DOIs)


A DOI, or digital object identifier, is an alpha-numeric string that is associated with a particular publication. It is similar to a URL (a uniform resource locator), the web link you see in your browser’s address bar. However, a DOI is more stable and will remain permanently attached to a publication.

The APA guidelines suggest that if a source has a DOI, you should include it in the citation, even if you did not access the source online.

Current Guidelines

The American Psychological Association follows the guidelines for DOIs provided by Crossref, the organization that helps publishers create consistent citation linking. A proper DOI should be in the following format (the letter being a variable number or letter):

Let’s take a closer look at the component parts of a DOI:

Note that every DOI will include the number 10 at the start of the prefix. The next number is at least four digits long and is associated with the registrant, a particular publisher or organization. The suffix consists of any number of letters and numbers.

When citing an online source with a DOI, add it at the end of your citation:

Wittles, Q. (2011). Freud and the art of doodling. Art and Psychology, 19, 22-33.

Make the link clickable if your writing is published online. Also, make sure you don’t add a period afterwards, as that may mess up the link.

How to Find a DOI

Most often you will find the DOI at the beginning of the article (look on the first page, above the title or in the header and footer).

If you’re using an academic database, you will also often find the DOI listed in the information for the article. In some cases, it may not be formatted correctly:

In this example, you would need to edit the link to get rid of “dx” and the library extension (“”):



Note that normally we would use https instead of http.

Older Methods

In the past, DOIs were sometimes formatted differently:


If you are citing the same DOI today, you’ll want to use the current format:


If you’re doubtful about the usefulness of DOIs, just take a DOI (not one from this article–they’re mostly made up) and copy it in your browser’s address bar (then press Enter). Alternatively, you can go directly to the DOI resolver at Knowing the DOI allows you to easily find the text it belongs to.

For more information about DOIs, please consult pp. 298-300 of the Publication Manual of the American Psychological Association (7th ed.).