Bolognese sauce is often thought to be a tomato-based meat sauce, but a true bolognese actually has very little tomato. It is also served with tagliatelle noodles instead of spaghetti, or tucked in between the layers of the green lasagna Bologna is famous for. Tagliatelle are similar to fettuccine, and are used because a broader noodle is a preferable cradle for a thick or heavy sauce. The ingredients in the authentic bolognese have even been officially named by the Accademia Italiana della Cucina: beef, pancetta, onion, carrot, celery, tomato paste, red wine, and milk.
This is not to say there are no variations, even in Bologna. Italians often use chopped pork or veal in their famous ragu, and chicken and goose liver may be added on special occasions. The onion, carrot, and celery can be cooked in butter as well as olive oil, and enrichments such as prosciutto, mortadella, and fresh porcini mushrooms when they are in season are also popular.