History of the NFL championship
Following its founding in Canton, Ohio in 1920, the NFL first determined champions through end-of-season standings through the 1932 season, switching to a playoff system in 1933 (a one-game playoff was required in 1932 to determine the champion). From 1933 to 1965, the NFL champion was determined by the season ending NFL Championship Game.
Source: Wikipedia — History of the NFL championship (CC BY-SA 4.0)