according ti wikipedia
A Chicago-style hot dog – as served in the U.S. city of Chicago, Illinois – is a steamed or boiled all-beef, natural-casing hot dog on a poppy seed bun, topped with mustard, onion, sweet pickle relish in fluorescent green, a dill pickle spear, tomato slices or wedges, sport peppers and a dash of celery salt — but no ketchup.

The style, once known as a "Depression sandwich," is generally acknowledged as having originated at Fluky's, a Maxwell Street stand, in 1929.

Chicago-style hot dogs are boiled or steamed to a temperature around 170 degrees Fahrenheit before adding the toppings. They can also be grilled, though these are referred to as "chardogs" in Chicagoland.

Chicago-style hot dogs with all the toppings are sometimes called "dragged through the garden" because of the unique condiments. However, Chicagoans traditionally shun ketchup because of the belief that ketchup, unlike the other condiments, overwhelms the flavor of the dog. Some vendors do not even carry ketchup.

The natural casing of the hot dogs give them their distinctive "snap." The most common dog weighs 2 ounces and is made by Vienna Beef.