A cheeseburger is a hamburger with cheese that has been added to it. Traditionally, the cheese is placed on top of the patty, but the burger can include many variations in structure, ingredients, and composition. The term itself is a portmanteau of the words "cheese" and "hamburger." The cheese is usually sliced, and then added to the cooking hamburger patty shortly before the patty is completely cooked which allows the cheese to melt. Cheeseburgers are often served with lettuce, tomato, onion, pickles, mustard, mayonnaise, or ketchup. In fast food restaurants, the cheese used is typically processed cheese, but there are variations, such as cheddar, Swiss cheese, mozzarella, blue cheese, and pepper jack.