The National Football League got underway in 1920 when it was known as the American Professional Football Association and began with 11 teams. Despite only 4 teams being able to finish the season the league kept going and continued to expand and contract wildly over that first decade. In 1922, the APFA was re-named as the National Football League.
Only a few franchises from those early seasons still exist today, the Chicago (now Arizona) Cardinals and Decatur Staleys (now Chicago Bears) were both teams in that inaugural season, the Green Bay Packers joined the APFA for their sophmore season in 1921. The New York Giants are the only other current-day franchise to particiate in that first decade, joining in 1925.
In 1966 the NFL announced they had reached a merger deal with the competing American Football League which would also establish a single "World's Championship Game" which would later officially be known as the Super Bowl. In 1970, the NFL and AFL completed their merger with the AFL teams forming the new American Football Conference and the existing NFL teams going to the National Football Conference.