Sports are an important part of culture in the United States. Historically, the national sport has been baseball. However, in more recent decades, American football has been the most popular sport in terms of broadcast viewership audience. Basketball has grown into the mainstream American sports scene since the 1980s, with ice hockey and soccer doing the same around the turn of the 21st century. These sports comprise the "Big Five". In the first half of the 20th century, boxing and collegiate football were among the most popular sports after baseball. Golf, tennis, and collegiate basketball are other spectator sports with longstanding popularity. Most recently, Mixed martial arts, has been breaking records in attendance and broadcast viewership for all combat sports. Based on revenue, the major professional sports leagues in the United States and Canada are the National Football League (NFL), Major League Baseball (MLB), the National Basketball Association (NBA), the National Hockey League (NHL), and Major League Soccer (MLS). At $16 billion in revenue, the NFL is the most profitable sports league in the world. The market for professional sports in the United States is roughly $69 billion, roughly 50% larger than that of all of Europe, the Middle East, and Africa combined. All these leagues enjoy wide-ranging domestic media coverage and, except for Major League Soccer, all are considered the preeminent leagues in their respective sports in the world. Although American football does not have a substantial following in other nations, the NFL does have the highest average attendance (67,254) of any professional sports league in the world. MLS has the second highest average attendance of any sports league in the U.S. (21,789), followed by MLB with an average of 18,900. Of these five U.S.-based leagues, all but the NFL have at least one team in Canada. Professional teams in all major sports in the United States operate as franchises within a league, meaning that a team may move to a different city if the team's owners believe there would be a financial benefit, but franchise moves are usually subject to some form of league-level approval. All major sports leagues use a similar type of regular-season schedule with a post-season playoff tournament. In addition to the major league–level organizations, several sports also have professional minor leagues, active in smaller cities across the country. As in Canada and Australia, sports leagues in the United States do not practice promotion and