What does the question mark at then end of a css include url do?

I've noticed that on some websites (including SO) the link to the CSS will look like:

<link rel="stylesheet" href="http://sstatic.net/so/all.css?v=6638"> 

I would say its safe to assume that ?v=6638 tells the browser to load version 6638 of the css file. But can I do this on my websites and can I include different versions of my CSS file just by changing the numbers?

Solution 1:

That loads all.css with a different query string so that if version 6637, for instance, is already cached on your machine, you'll get the new one (6638). Changing that number (in this case) will not give you a different file.

This is just a cache trick so they can send the file down with no expiration (i.e. you never have to ask for it again), because when it does change, the "file name", changes.

That said, you could make it so you load a different version based on the query string parameter. Doing so would be slightly non-trivial and akin to how you get different questions when you pass a different question ID to the URL of this page.