Professional American Football Leagues started with the National Football League (NFL) in 1920.
Rivals to it included numerous Leagues called the American Football League until one in 1960 became a success, which is why the Super Bowl is played – between the winners of the NFL & AFL (now merged – with the two leagues called the National Football Conference & American Football Conference.)
Since then there has been the World Football League (1973-1974), United States Football League (1983-1985) and XFL (2001).
There has been numerous Pro Development Leagues also from the Continental Football League (1965-1969), World League of American Football/ NFL Europe (1991-2007), & United Football League (2009-2012)
Major Leagues (AFL Era):
Rival Major Leagues (Modern Era):
World/Continental Pro Leagues (Development Leagues):