Judging by the ever-present lines at neighboring Pat’s and Geno’s—two historic rival institutions in a not-so-scenic stretch of the City of Brotherly Love—no sandwich arouses regional debate with quite as much intensity as the Philly cheesesteak. This seemingly simple sandwich of thinly-sliced ribeye, melted cheese (more on that in a minute), and optional vegetation (onions, peppers, mushrooms, etc.) has inspired not only fierce loyalties, but also its own particular rapid-fired patois as well. (“One whiz with” = One cheesesteak with Cheez Whiz and fried onions; “One provolone without” = One cheesesteak with provolone cheese without fried onions, etc.)

But where does this beloved sandwich come from? What stories lurk beneath its processed-cheese shell, nestled between layers of frizzled beef? Like so many iconic regional foods, the cheesesteak origin story has been conflated with myth and braggadocio so many times that it’s difficult to pin down a definitive truth. Even so, there are luckily a few key points that remain uncontested in the sandwich’s history.

To find out more, we spoke to Carolyn Wyman, food writer, Philadelphia food tour guide, and author of The Great Philly Cheesesteak Book, a veritable font of information on the history and evolution of the sandwich. In the process, we learned how the humble creation rose to prominence out of the Depression Era at the hands of its savvy inventor-come-marketer, and eventually grew into the icon it’s considered today.

Here’s an illustrated history of the evolution of the Philadelphia cheesesteak.