The NFL was formed in 1920 as the American Professional Football Association (APFA) before renaming itself the National Football League for the 1922 season. After initially determining champions through end-of-season standings, a playoff system was implemented in 1933 that culminated with the until 1966. Following an agreement to merge the NFL with the rival American Football League (AFL), the Super Bowl was first held in 1967 to determine a champion between the best teams from the two leagues and has remained as the final game of each NFL season since the merger was completed in 1970. Today, the NFL has the highest average attendance (67,591) of any professional sports league in the world and is the most popular sports league in the United States. The Super Bowl is also among the biggest club sporting events in the world, with the individual games accounting for many of the most watched television programs in American history and all occupying the Nielsen's Top 5 tally of the all-time most watched U.S. television broadcasts by 2015. The NFL is the wealthiest professional sports league by revenue, and the sports league with the most valuable teams.