Opening Local HTML Files using HTML and Javascript

I have a project I'm working on and I am trying to do file organisation with it

I want to open a html file in a folder before it.

Here is an example of the folder tree I am creating:

• Main Folder
  • Game_Folder
    • Rules

Each folder has a HTML, CSS and JS file in it (For organisation reasons). But I want to be able to open 'Game_Folder.html' from the 'Rules.html'. I know how to do it from the other way, I just cannot figure out how to do it backwards.

I cannot use a full directory (c:/) as it will be operated on a few different computers. So the file location must be in local file format (if that makes sense).

Thanks in Advance

-J