This is a tracking category. It is used to build and maintain lists of pages—primarily for the sake of the lists themselves and their use in article and category maintenance. It is not part of the encyclopedia's categorization scheme. More information: This category is hidden on its member pages—unless the corresponding user preference (Appearance → Show hidden categories) is set. These categories are used to track, build and organize lists of pages needing "attention en masse" (for example, pages using deprecated syntax), or that may need to be edited at someone's earliest convenience. These categories also serve to aggregate members of several lists or sub-categories into a larger, more efficient list (discriminated by classifications). Typically, tracking categories are automatically populated by templates invoked in pages or by the MediaWiki software (see Special:TrackingCategories for an overview of the latter).

Administrators: Please do not delete this category as empty! This category may be empty occasionally or even most of the time.

This is a tracking category for CS1 citations that use |doi= with an assigned value that is known to be free-to-read but is not flagged as such with |doi-access=free.

Most (although not all) cases in the category can automatically be flagged with |doi-access=free by clicking here to activate User:Citation bot. Exception are pages with ((Academic peer reviewed)) or ((Cite Q)) on them, where the update needs to be done in Wikidata.

cs1|2 maintains a list of DOI registrants (the portion of a doi prefix that follows the 10. directory indicator). When a DOI registrant matches a registrant listed in the cs1|2 list, the module adds this category to the citation.

Pages in this category should only be added by Module:Citation/CS1.

Pages with this condition are automatically placed in Category:CS1 maint: unflagged free DOI.^[a]

By default, Citation Style 1 and Citation Style 2 error messages are visible to all readers and maintenance messages are hidden from all readers.

To display maintenance messages in the rendered article, include the following text in your common CSS page (common.css) or your specific skin's CSS page and (skin.css).

(Note to new editors: those CSS pages are specific to you, and control your view of pages, by adding to your user account's CSS code. If you have not yet created such a page, then clicking one of the .css links above will yield a page that starts "Wikipedia does not have a user page with this exact name." Click the "Start the User:username/filename page" link, paste the text below, save the page, follow the instructions at the bottom of the new page on bypassing your browser's cache, and finally, in order to see the previously hidden maintenance messages, refresh the page you were editing earlier.)

.mw-parser-output span.cs1-maint {display: inline;} /* display Citation Style 1 maintenance messages */

To display hidden-by-default error messages:

.mw-parser-output span.cs1-hidden-error {display: inline;} /* display hidden Citation Style 1 error messages */

Even with this CSS installed, older pages in Wikipedia's cache may not have been updated to show these error messages even though the page is listed in one of the tracking categories. A null edit will resolve that issue.

After (error and/maintenance) messages are displayed, it might still not be easy to find them in a large article with a lot of citations. Messages can then be found by searching (with Ctrl-F) for "(help)" or "cs1".

To hide normally-displayed error messages:

.mw-parser-output span.cs1-visible-error {display: none;} /* hide Citation Style 1 error messages */

You can personalize the display of these messages (such as changing the color), but you will need to ask someone who knows CSS or at the technical village pump if you do not understand how.

Nota bene: these CSS rules are not obeyed by Navigation popups. They also do not hide script warning messages in the Preview box that begin with "This is only a preview; your changes have not yet been saved".