For the most part, major global airlines slot cleanly into one of two categories. They are either international legacy carriers, airlines which primarily service business travelers, or low-cost airlines, carriers which primarily aim to serve customers with discounted tickets and no-frills offerings. Over the past four decades, the commercial aviation industry has gradually moved towards a system in which only these two kinds of airlines are really allowed to exist. Premium travelers fly with legacy airlines, and those in search of a discount tend to fly with low-cost carriers.