How is the root folder of my website project determined?

On my local hard drive my website project is located at E:\websites\usr. However, PG doesn’t require this full path; it somehow knows that the project root is “usr”. Then, for example, a mathjax script path can be specified as rather than specifying .

My question is, “How does PG know that “usr” is the project root?” This info must be stored somewhere (I couldn’t find anything in “Settings”).

Don C.