include paths...

Ok, I'm not understanding how this works.

I have a file called index.php



Fine, that works from the root of the web directory.
As does this images/index.php


But what happens when this is the case includes/auth.php



How does index.php and images/index.php know how to deal with logger, and auth.php for that matter?

I guess I want to know how to set these without including the full unix path.