1. There are only three completely essential ingredients to a Philly cheesesteak: thinly-sliced rib eye steak, melted cheese, and a long Italian roll (preferably Amoroso's). Grilled onions, while in essence the fourth ingredient, are optional.
2. Additional toppings such as mushrooms, tomato sauce, and peppers are commonly ordered but in no way define an authentic cheesesteak.
3. If you ask for lettuce, tomatoes, and mayo on your cheesesteak, that is not a proper cheesesteak but rather a cheesesteak hoagie.
4. Contrary to what you've heard, Whiz is not the one-and-only true authentic cheesesteak cheese. If you really want to get "old school" about it, provolone is THE cheese. Whiz is by far the best-selling cheesesteak cheese. American cheese ranks second, ahead of provolone. John Kerry infamously tried to order a cheesesteak with Swiss. Such behavior is not acceptable and may, in fact, be illegal.
5. The steak on a cheesesteak, after being thinly sliced, may be chopped (a la Pat's King of Steaks) or not chopped (a la Geno's Steaks).
6. The first cheesesteaks, sold by Pat and Harry Olivieri in the early 1930s, actually did not have cheese on them. They contained only chopped steak and grilled onions. "Cocky Joe" Lorenza, a manager at one of the Pat's locations, later added provolone cheese.
7. The proper way to order a Philly cheesesteak: for a steak with Whiz and onions, order "Whiz Wit". For a provolone steak without onions, order "Provolone Witout".