A cheeseburger is a hamburger topped with cheese. Traditionally, the slices of cheese is placed on top of the meat patty. The cheese is usually added to the cooking hamburger patty shortly before serving, which allows the cheese to melt. Cheeseburgers can include variations in structure, ingredients and composition. As with other hamburgers, a cheeseburger may include toppings such as lettuce, tomato, onion, pickles, bacon, mayonnaise, ketchup, and mustard.