A patty melt is a type of hamburger consisting of a hamburger patty and Cheddar or Swiss cheese between two slices of bread (traditionally rye, though sourdough is sometimes substituted). In other places, especially in the U.S., a patty melt can consist of only the patty of a hamburger, with cheese, on a single piece of toast/bun. The hamburger is then fried with butter on a frying pan so that the cheese melts thoroughly. A patty melt, being a toasted and pressed sandwich, may also be considered a type of panini or a variation on the grilled cheese. It's unclear when the actual patty melt was invented, but records exist of them being served as early as the 1940s. They embellish on the traditional cheeseburger, and further take the burger back to its roots by being served on bread versus a bun.