History of the National Football League on television
NBC was the first major television network to cover an NFL game, when on October 22, 1939, it broadcast a game between the Philadelphia Eagles and the Brooklyn Dodgers; the network was still only in its infancy, with only two affiliates, the modern day WRGB (now a CBS affiliate) in Schenectady and W2XBS in New York City. Portions of that game still survive via films, but the film is not footage from the telecast (recordings of television broadcasts did not begin until 1948). The use of a sideline camera, the sole camera used in the 1939 broadcast, would become the standard for all future NFL broadcasts until 2017; the angle is particularly suited for estimating yardage, compared to more mobile camera angles that began to appear in the 21st century.
NFL on CBS
When the AFL and the NFL officially merged in 1970, the combined league divided its teams into the American Football Conference (AFC) and the National Football Conference (NFC). It was then decided (officially announced on January 26, 1970) that CBS would televise all NFC teams (including playoff games) while NBC would carry games from all AFC teams. For interconference games, CBS would broadcast them if the visiting team was from the NFC and NBC would carry them when the visitors were from the AFC. This was in line with the NFL television blackout rules of the time, meaning that every televised game of a local NFL team would be on the same channel (at the time, home games were banned from local television regardless of sell-out status, while road games are required to be aired in the teams' primary media markets, and select neighboring markets as well, even if it is not the most popular team in the market). The two networks also divided up broadcast rights to the Super Bowl on a yearly rotation.