The URL structure defines a hierarchy similar to a filesystem's hierarchy of subdirectories or folders. The segments of a URL are separated by slash characters ("/"). When navigating the URL hierarchy, the final segment of the URL (i.e., everything after the final slash) is similar to a file in a filesystem. The other segments of the URL are similar to the subdirectories and folders in a filesystem.

When resolving relative URLs (see the answer to the previous question), the browser's first step is to strip everything after the last slash in the URL of the current document. If the current document's URL ends with a slash, then the final segment (the "file") of the URL is null. If you remove the final slash, then the final segment of the URL is no longer null; it is whatever follows the final remaining slash in the URL. Removing the slash changes the URL; the modified URL refers to a different document and relative URLs will resolve differently.

For example, the final segment of the URL http://www.mywebsite.com/faq/html/ is empty; there is nothing after the final slash. In this document, the relative URL all.html resolves to http://www.mywebsite.com/faq/html/all.html (an existing document). If the final slash is omitted, then the final segment of the modified URL http://www.mywebsite.com/faq/html is "html". In this (nonexistent) document, the relative URL all.html would resolve to http://www.mywebsite.com/faq/all.html (another nonexistent document).

When they receive a request that is missing its final slash, web servers cannot ignore the missing slash and just send the document anyway. Doing so would break any relative URLs in the document. Normally, servers are configured to send a redirection message when they receive such a request. In response to the redirection message, the browser requests the correct URL, and then the server sends the requested document. (By the way, the browser does not and cannot correct the URL on its own; only the server can determine whether the URL is missing its final slash.)

This error-correction process means that URLs without their final slash will still work. However, this process wastes time and network resources. If you include the final slash when it is appropriate, then browsers won't need to send a second request to the server.

The exception is when you refer to a URL with just a hostname (e.g., http://www.mywebsite.com). In this case, the browser will assume that you want the main index ("/") from the server, and you do not have to include the final slash. However, many regard it as good style to include it anyway.