It is my understanding that novels should generally start right off with the protagonist. The story is about the protagonist, after all, not something else (This obviously changes a bit if the PoV is not the protagonist, but that is beside the point). It is also my understanding (through experience as well as being informed) that too much telling will generally bore the reader, while showing will not. This is also beside the point, however.
I recently started to re-read Harry Potter and the Sorcerer's (Philosopher's) Stone. Imagine my surprise and confusion, when I discovered that the first chapter barely contained Harry at all. He shows up in the last two pages, but was only a 1-year-old sleeping baby. My confusion deepened as I realized that the first half of the chapter was almost pure telling.
Harry Potter was clearly successful. This means that a book clearly does not need to open with the protagonist. My question is this: Was J. K. Rowling one of those authors who knew how to write an exception to 'the rules?' Or is my above assumption (that you need to start with the protagonist) wrong? And if it is, why is it, and how can you be successful when you do not start with the protagonist?
Note: I realize that no writing 'rules' are set in stone, and that there are likely exceptions to many (if not all) of them. Some authors know how to bypass the rules, but most do not. At least such is my understanding.