Relative URLs have been mentioned on several occasions earlier in the chapter, and they deserve some additional attention at this point, too. The reason for their existence is that on almost every web page on the Internet, a considerable number of URLs will reference resources hosted on that same server, perhaps in the same directory. It would be inconvenient and wasteful to require a fully qualified URL to appear in the document every time such a reference is needed, so short, relative URLs (such as ../other_file.txt) are used instead. The missing details are inferred from the URL of the referring document.

Because relative URLs are allowed to appear in exactly the same scenarios in which any absolute URL may appear, a method to distinguish between the two is necessary within the browser. Web applications also benefit from the ability to make the distinction, because most types of URL filters may want to scrutinize absolute URLs only and allow local references through as is.