Yes, it has to do with TEMPLATEPATH and changing it with STYLESHEETPATH in some cases and having to work with, what you say, other files than the one I am working with, which it depends on in some way. So I see, it isn’t “simply using a file with the same name” as the codex says in some cases… I have seen in the internet many resources and examples about using the style.css and functions.php in child themes but almost none at all about using template files.