Pro Football Hall of Fame, American Football Wiki
History of American football - Wikipedia
Once a fierce Chicago Bears rival, what ever happened to the NFL's Rock Island Independents? - The Athletic
Founders & Stewards': Wilf Family Funds New Pro Football Hall of Fame Exhibit
NFL Countdown: American Professional Football Association is formed
Sept. 17, 1920 -- The Founding of the NFL
NFL's 1st decade was the 1920s, when teams came and went
The NFL: 100 Years of football - Los Angeles Times
Chronology of Professional Football