DOIs and URLs

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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.