<P> A citation ideally includes a link or ID number to help editors locate the source . If you have a URL (web page) link, you can add it to the title part of the citation, so that when you add the citation to Wikipedia the URL becomes hidden and the title becomes clickable . To do this, enclose the URL and the title in square brackets--the URL first, then a space, then the title . For example: </P> <P> For web - only sources with no publication date, the "Retrieved" date (or the date you accessed the web page) should be included, in case the web page changes in the future . For example: Retrieved 15 July 2011 or you can use the accessdate parameter in the automatic Wikipedia: refToolbar 2.0 editing window feature . </P> <P> You can also add an ID number to the end of a citation . The ID number might be an ISBN for a book, a DOI (digital object identifier) for an article, or any of several ID numbers that are specific to particular article databases, such as a PMID number for articles on PubMed . It may be possible to format these so that they are automatically activated and become clickable when added to Wikipedia, for example by typing ISBN (or PMID) followed by a space and the ID number . </P> <P> If your source is not available online, it should be available in reputable libraries, archives, or collections . If a citation without an external link is challenged as unavailable, any of the following is sufficient to show the material to be reasonably available (though not necessarily reliable): providing an ISBN or OCLC number; linking to an established Wikipedia article about the source (the work, its author, or its publisher); or directly quoting the material on the talk page, briefly and in context . </P>

Where does the isbn go in a citation